Well, of course the answers to this question will be largely opinion, as is mine.
I have been spoiled by the performance of my trusty E-MU0404 PCI card. For a long time, I was not particularly impressed by the performance of the USB interfaces I had seen. Generally, the minimum latency was just outside of what I considered tolerable.
With contemporary machines, this seems to have improved some.
I really like the look and specs on the Focusrite Scarlet series, but I am currently working with a Vista machine, and they suggest a minimum W7 platform. That said, with the appropriate platform, I would look very hard at the Scarlet series.
I have limited faith in USB power, especially when phantom power is involved. It may be fine, but I'll take a wall wart any day. 500 va only goes so far.
For me, that ruled out the 2i2 and 2i4. However, the 6i6 offers external power, and is a very attractive unit.
Sweetwater currently has a killer deal on the 6i6 And I am a big fan of their 3 payment plan.
Ultimately, I went with the AKAI EIE PRO. Reading the reviews, it became apparent the the people that were happy with it knew what they were doing. It was a bit of a long shot, but offered two things I found very attractive.
Phantom power could be applied to the four audio inputs in pairs (1/2 and/or 3/4), and it has a built-in USB bus for multiple MIDI controllers( which was huge for me). Initially, it was a little shaky, but after messing with buffers and driver modes, it turned out to be a very decent unit. The pre's seem respectable. Reasonably clean, and plenty hot.
I record a lot of MIDI live, and use amp sims for my guitars, so low latency performance is paramount. To that end it has not let me down.
Again, at the outset, it was not a happy camper, but once it settled down, it has proven to be a good choice.
M-Audio has a similar unit in the wings, but theirs has not been released yet.
I'm not saying it's the best, but for relatively inexpensive hardware($249 US), it is a viable contender.
post edited by RobertB - 2013/11/28 20:31:49