I'm currently in the process of upgrading my studio, from my DAW desk furniture to my computer's OS (bought a larger SSD drive and will install Windows 10) to an eventual upgrade to SONAR Platinum, but the heart of the upgrade is a new audio interface.
 
For the past 15 years or so, I've been using an ESI Waveterminal 192M PCI interface, which has served me well, but the no-longer-updated drivers seem to freeze my Sonar X3e every so often on Windows 7, and I also realized it's time to grow up and get a non-PCI interface.
 
What started out as intending to buy a Presonus Studio 192 led me on a search for other interfaces. My requirement was to find a new interface that was rackmountable, had low-latency drivers, was reliable and also had the flexibility to have ADAT/SPDIF/Word Clock/etc. I wanted either USB-3 or Firewire (I still have a compatible Firewire card). For some reason, I'm not fully sold on USB2, mainly because I'm afraid all my other USB peripherals will get in the way of A USB-2 interface's performance, plus most of my PC's USB jacks are USB-3, and USB-2 interfaces reportedly only work well when plugged into a USB-2 specific jack.
 
I've settled on two contenders, the Zoom UAC-8 and the Focusrite Saffire Pro 40. Here's my rationale:
 
Zoom UAC-8
https://www.zoom-na.com/products/production-recording/audio-interfaces/zoom-uac-8-usb-30-audio-converter

PROS:
-USB-3 interface
-24 bit/192khz
-Reportedly very low latency drivers
-New model
- So far positive reviews
 
CONS:
-Wall wart power
-Zoom lacks experience in audio interface market
-Not familiar with Zoom interface preamp quality
-Very little solid performance reviews on this product
-Drivers might still need more updates to solve issues
 
Focusrite Saffire Pro 40
https://us.focusrite.com/firewire-audio-interfaces/saffire-pro-40/artists-reviews#media-reviews

PROS:
-Firewire speed and longtime reliability
-Focusrite preamps has great reputation in audio interface market
-Reportedly very low latency drivers
-Stable, developed drivers
 
CONS:
-Older product
-Firewire is dying out
-Only 24bit/96khz
 
Can any experienced users of either chime in and help my make my decision? 

I'd pick the zoom of the two.  Otherwise, I'm pretty happy w/ a TASCAM US 20x20.  Sounds great, has all the pro features (WC, ADAT, Spdif), a good ole fashioned soft mixer board layout, the back layout printed on the top of the unit (nice for home studios w/o racks), plus, it sounds great (not as nice as the UH-7000 but close enough for 8x8 work).  I was really surprised by how much you get for $400 or so.
 
Drivers have some latency, but they continue to work on them (they should be ripe for a newer version - all their new US line shares drivers and they have driver-writers in house).  If latency is your most needed feature, you'll probably want to skip it (or wait for newer drivers), but otherwise, it is a really good deal.

AT
I'd pick the zoom of the two.  Otherwise, I'm pretty happy w/ a TASCAM US 20x20.  Sounds great, has all the pro features (WC, ADAT, Spdif), a good ole fashioned soft mixer board layout, the back layout printed on the top of the unit (nice for home studios w/o racks), plus, it sounds great (not as nice as the UH-7000 but close enough for 8x8 work).  I was really surprised by how much you get for $400 or so.
 
Drivers have some latency, but they continue to work on them (they should be ripe for a newer version - all their new US line shares drivers and they have driver-writers in house).  If latency is your most needed feature, you'll probably want to skip it (or wait for newer drivers), but otherwise, it is a really good deal.

Just to clarify, your post was generally talking about the Tascam unit, correct?

No experience of the Zoom product but I run a Saffire Pro 40 and am very happy with it.  It interfaces via ADAT to my O1V which I use for my hardware synths.  The mixer interface, Mix Control, software takes a bit of time to get your head round but once you have grasped the basics it is very flexible.

The main thing is to pick the one that fits in with your recording requirements and workflow.

Never been a big fan of Zoom for some reason. There stuff always seemed like toys to me. Focusrite has a stronger history in the audio world, and I've been happy with their products that I own. Personally, I would go with the Focusrite. That said, I've never owned any Zoom products, and my opinion of them have been based on appearance and my limited use of a couple products. The Zoom interface might be great, for all I know.

I would throw caution at the Zoom just from a potential driver point of view. I have a Zoom R8 that I picked up recently. In Windows 7 the driver won't allow application sharing (can't watch Youtube and have Sonar using it at the same time). In Windows 10, Ableton Live won't even open it. I have a cheap Behringer unit that will allow device sharing etc but it only has 16-bit DAC vs 24-bit that I would prefer.
 
Now I know it is not an identical device but I would be confirming compatibility with someone who has one before you buy it. I am now looking at a new interface myself because of this problem.

I finally bought an interface; it was a Focusrite Scarlett 18i20 2nd Generation. It's USB 2.0, but so far so good. The latency is really good for my softsynth needs.

hi, Zoom UAC-8 driver is unstable and crashes when using Albleton Live (windows 10 64bit)
see video here: hxxps://www.youtube.com/watch?v=A9Yo_KOvD2s