I just purchased a Saffire PRO 24 DSP last month and was my first exposure to FW. It would be nice to be able to do the same test gswitz did above with it to get an "apples to apples" comparison - as it stands, it makes my old USB 2.0 interface look like a relic. I run the same 48 buffers ASIO and am getting:
 
Firewire
Input: 1.5msec, 65 samples
Output: 2.1msec, 94 samples
Round Trip: 3.6msec, 159 samples
 
Many items coming online with USB 3.0 aren't delivering the speed that USB 3.0 is capable of (or even close depending on what it is), so I have been disappointed with its adoption in general. Even "waiting" for Focusrite to adopt USB 3.0 could be a crap shoot.
 
I agree with chuckebaby... I narrowed in on Focusrite while searching, then threw the question into this forum and not a single person said anything bad about them. As far as Saffire vs Scarlett, it may simply boil down to personal preference as they are the same brand name, but I went with FW more because it is dedicated to the interface and "on paper" is superior to USB 2.0. (plus the feedback from all the folks kind enough to chime in on this thread, many of whom have the Saffire 40).

I have the RME UFX and the Tascam FW1884.
Now admittedly the 1884 is "orphanware", but the UFX is not.
In doing a little research at the RME site, they feel the USB port is "faster" on most systems than the FW port (UFX has both); seems there are more "hidden" buffers with the FW.
The 1884 is very "picky" about the chipset, but again has been orphaned so maybe not the "best" yardstick.
You do (or should, we are back to the UFX here) make sure the USB port its on is not "shared" (device manager).
Jim Roseberry walked me through all that...
Just my nickle98,
Tom

Obviously the question of FW vs. USB will be different depending on the needs of each user. I can only speak for my own needs and the more I think about it....I don't think I would notice a difference between the two formats in my home studio. I say this because even though my PC will be equipped with several USB 2 and 3 ports, I won't be having anything utilizing them other than a portable HD for project / file backup purposes only. And that would only be plugged in AFTER tracking, processing, etc. So really, at least on my system, a USB interface wouldn't suffer from shared USB bandwidth. Not to mention I will only be recording one, maybe two tracks at a time.

So now you might be asking yourself why I would want or need an in interface with 8 inputs if I'm only recording one track at a time?? I tend to be a "set and forget" kind of guy. I like having a dedicated input for each of my instruments with the appropriate levels set and ready to go so that I can just plug in and play without having to make any adjustments. But that's just me.

Firewire is faster on sustained throughput, but irrelevantly so for many applications. And certainly when only tracking a few tracks at the same time. I would definitely go with USB for future compatibility.

There isn't a year that goes by without a USB patch from ms. And I have seen several motherboards that have screwed up the USB standard so it suffers from same issue as FireWire. Regardless there are two very reliable chipsets with firewire as pointed out earlier. Swings and roundabouts always. Things will be clearer in a year.

Yet every computer has a USB port and will for a long time to come. Firewire has been on its way out for a while.

Mojo3432

So now you might be asking yourself why I would want or need an in interface with 8 inputs if I'm only recording one track at a time?? I tend to be a "set and forget" kind of guy. I like having a dedicated input for each of my instruments with the appropriate levels set and ready to go so that I can just plug in and play without having to make any adjustments. But that's just me.

That's exactly why I chose the Scarlett 18i6 - pure convenience.  I haven't gone above two simultaneous input tracks, but for that purpose it works very well.   The drivers are stable and I can typically sit in the 1-3ms range when recording and go a bit higher when mixing - very little in the way of pops and clicks... ever.  
 

I like the feedback on the Scarlett/Focusrite I/O's - this is good to know.  And yes, eventually if Firewire does not produce a standard which competes with USB 3.0 - then I am totally onboard USB 3.0 - especially given the reality that every device out there will need to employ it as at least an alternative, in order to sustain any kind of sales.  But that being said here is my experience on the upgrade pathway over the last few months inside a Gen 4 i7 box about 3 months old.  I would stay away from USB 2.0 sound.
 
I.  Firewire 400 PCI-e (Non TI chipset) to Firewire 400 MOTU 838 mkIII - Stable and clear but drivers would degrade after 1 - 2 hours and with any power saver or screen saver activity.  Livable.....but ehhh.....
 
II.  USB 2.0 Native (Intel(R) 8 Series/C220 Series USB Enhanced Host Controller - 8C26) to USB 2.0 MOTU 828 mk III Hybrid - Stable for 10 to 40 minutes and began to have ASIO integrity loss without a triggering event.  ASIO drivers had to be continually reset.  Could not employ over 8 tracks or 6 active VST 2 live effects while drivers were functioning without significant crackling and popping.  Windows 8.1 upgrade introduced a system event which would corrupt drivers totally.  Could not find or eliminate this after working for weeks with Sonar and MOTU.
 
III.  Firewire 800 PCI-e (TI Chipset) to MOTU 828 mk III - Completely stable and unaffected by system events, no ASIO driver loss observed thus far, with the system on for 8 hours continuous (never attained that before). Fastest total system speed I have had thus far :
 
2.4 msec - 226 samples
2.9 msec - 276 samples
5.2 msec - 502 samples  (96 samples per millisecond duplex compared to 44 duplex for Firewire 400 and 53 half-duplex theoretical for USB 2.0 -which I never actually could use)
 
But remember it is not JUST the line transmission speed but the inability to prioritize instruction packets versus sound, when operating in a half-duplex communications protocol with USB 2.0 and the degrading of drivers with a non-TI chipset.  The Windows 8.1 thing seems to have disappeared with the new Firewire 800 PCI-e TI Chipset card.  This despite my computer only being 3 months old; the native USB drivers/chipset seemed to have problems.  I have no idea why (maybe the chipset, maybe USB 2.0 drivers used), but I am glad it is gone.  ...whew.  

TStranger, that is EXACTLY what I have been dealing with.  It is absolutely maddening having to deal with the interruptions, fussing with the drivers, etc.  After only 15 minutes of working properly, it would ALWAYS start to act up where I would have to continually switch back and forth between WDM and ASIO drivers to get the problem solved.  And even that would only take care of it for perhaps an hour.  
 
I am however in the process of putting together a new system with a TI chipset for FW and will most likely end up with a Focusrite Saffire Pro 40.
Focusrite says the two should play well together.  Here's hoping.

Always good to hear from multiple sides. Though your results perhaps say more about MOTU than about USB. So far I've used succesfully on USB:
1. Avid/M-Audio Fast Track Pro
2. RME UCX
3. Edirol UA25EX
4. Reloop DJ controller/interface
5. Behringer X-32
6. Pioneer DDJ-T1
7. NI S4 mk2
8. Korg Kronos using Asio4All
 
No issues with any of them other than the odd driver update. It's worth noting that not all USB ports are identical. Some will be attached to a hub inside and some supply more/better power than others. This is especially true for laptops and very impprtant when using bus-powered devices.