With RME you get VERY stable drivers and low latency (I have not had a single problem with my RME in over a year). It also has Totalmix, which easily lets you route everything everywhere. And it has DSP reverb and compressor, which allows you to hear reverb or compression when tracking (it will not record those effects). The UFX has 4 preamps, and a lot of extra ins (analog, spdif and ADAT).
The UA Apollo (IIRC) has two preamps, and less inputs / outputs, but you will have access to unison preamps, which can emulate other preamps (most of them will have to be bought). I have an UAD pcie card (and a UA LA 610mkII), so I can vouch for the quality from UA.
It all comes down to what you need (input / output vise), want and can afford
I think you will be very happy either way, but I personally would recommend RME. Mostly because I have used it for a while, and would never change something that has been that stable, and sounds that good.
Other may feel different about my statement.
Best of luck.

THX a Lot 

First, no comparison between appolo twin and RME UFX - Have both and the UFX has ton of I/O the Twin barely half the inputs and harldly no outputs.   Unless you have the UAD console plugins to take advantage of the Unison Technology which is why I got the twin, UFX and Apollo twin not even in same ballpark.    UFX IS AMAZING UNIT AND HAS TONS OF I/O options  30 in and 30 out PLUS direct USB recording too for safety.   TWIN has at most 10 in and only a few out and NO DIGITAL OUTS!  Only Analog.   RME UFX HAS Optical Adat/Spdif and AES OUT besides 8 analog outs.  Quality wise, RME and UA are both equal and top shelf
 
NOW FOR TWIN USB USERS! IF YOU DO NOT HAVE BUILT IN USB 3 INTO YOUR MOTHERBOARD, THERE ARE ONLY A COUPLE OF USB 3 ADAPTERS THAT WILL WORK.  I HAVE ONE THAT DID NOT AND AFTER SEARCHING THE UAD SITE I FOUND  INATECK  KT 4005 AND THEN THE TWIN WORKED PERFECTLY!   HOWEVER, I HAVE A LAPTOP AND A DOCKING STATION THAT ACCEPTS PCIE CARDS SO I WAS  ABLE TO USE THAT!  I HAVE A SIIG EXPRESS 34 COMING TOO AND WILL TEST THAT OUT BUT ITS NOT ONE OF THE ONES RECOMMENDED BY UA SO WE'LL SEE.   IF YOU DON:T HAVE NATIVE USB 3, THAT EASILY COULD ACCOUNT FOR SOME AUDIO ENGINE ISSUES.  BTW, BEFORE I GOT THE PROPER ADAPTER, THE APOLLO CONSOLE DID WORK BUT SONAR DID NOT GET ANY OF THE AUDIO . 

972swone
Ok and you can tell me what's difference with ?
i can't decide me "Apollo twin " or  Rme UFX

 
Huge difference in features... and in cost  
Take stock of your current situation/goals... and let that determine your needs.
 
The RME Fireface UFX offers lower round-trip latency than the Twin Duo USB.
The UFX can go down to 4.3ms total round-trip latency at a 48-sample ASIO buffer size (44.1k).
That's the best you'll find in any USB audio interface.
Best part is that it's rock-solid at that 48-sample ASIO buffer size.
As long as your machine can sustain the load, audio is completely clean.
 
If you don't need the more extensive I/O from the UFX... and you want the ability to run UAD plugins (pre and post recording), then the Apollo Twin Duo USB is a good choice.
 

It seems to me the comparison, feature wise, should go between the RME UCX and the Apollo Twin. The UCX is very similar to the UFX but
1. Has only two preamps, not four
2. Has only one ADAT I/O, not two
3. Has no AES/EBU
4. Doesn't do direct to USB recording

Apart from those features the sound quality, Totalmix flexibility and performance is on par with the UFX. It's a lot cheaper though and feature wise more comparable to the Twin than the UFX is.

EDIT:
For the sake of full disclosure, I use a UCX and love it. Can only corroborate the comments about stability, sound quality, latency and flexibility.

I have to agree with Sander. I have the Ucx myself, and love it.

And remember you need to use cables that are specified for use with USB 3.0.

Did UAD ever address the compatibility issues with WIN 10? I remember a user on the UAD forum stating that he had to roll back to an earlier version of UAD to get it to work with WIN 10.
 
This has kept me from upgrading my WIN 8.1 machine to WIN 10... would love to hear an update if there is one.
 
Thanks,
Jim
 
PS - OP, what do you have your latency set to?  Too low and the audio engine will stop/stutter in SPLAT.

The RME is both USB and Firewire, while the UA is USB only. I am using mine over USB with zero issues at low latency (2 - 5 ms) on quite large (in my book) projects.
Best of luck to you on your purchase, and as said earlier, you will be happy either way

jimkleban
Did UAD ever address the compatibility issues with WIN 10? I remember a user on the UAD forum stating that he had to roll back to an earlier version of UAD to get it to work with WIN 10.
 
This has kept me from upgrading my WIN 8.1 machine to WIN 10... would love to hear an update if there is one.
 
Thanks,
Jim
 

Hi. I am running the latest UAD software on my UAD2 pcie (solo) on Win10 X64 with no problems. When I was on Win7, I could not use the latest version (needed to go back to version 7.6 IIRC), but after Win10 all is well on my end.
All the best.