Now that I've used about a dozen of the entry and mid-level interfaces from the common brands, I've been amazed at how similar they all are. There are differences in how they each sound, but those differences are really subtle. Converter quality and preamp quality is pretty consistent across brands. They're all aiming for the same thing: clean, low-distortion, enough headroom and enough gain. Focusrite's Scarlett is the only one I can remember that even has much a "sound" of its own. They all sound kind of boring, frankly. And I think that's what we want in an interface. These things are made with the aim to please a lot of people, and to be relevant in a lot of different situations. Any company that would make their interface sound remarkably different would probably be committing marketing suicide.
Anyways, the main differences that I've noticed are in a few areas:
- connectivity to the computer. USB is by and large the most popular, but some brands offer Thunderbolt, PCIe, and the older FireWire connectors
- Windows drivers. RME has been the most impressive to me in this regard, and everybody else's drivers pretty much tie for 2nd place. Your mileage may vary, depending upon your specific computer. Everything I've used over the last few years has been rock solid, and pretty similar in terms of latency and stability.
- bells and whistles. This would be things like 3rd gen Focusrite's Air button, SSL's 4K button, MOTU's fancy meters, UA's on-board DSP, Behringer's pads and inserts...things like that
- software bundles. This can really sweeten the deal if you're buying an interface for music production. NI's virtual instruments, Focusrite's Red plugins, SSL's compressors, they're all great, and make for a nice treat on day 1 of plugging in your new interface. And pretty much everything comes with a "lite" version of a DAW these days as well. I think that this is where Behringer is the big loser, as they just point you to a freeware DAW like Tracktion and they don't come with any cool plugins to play with.
- headroom and gain. Focusrite used to be the big loser in this category, but they've made big strides in their 2nd and 3rd gen Scarletts. Steinberg didn't impress me with their UR22 mkII in this regard either. Behringer had the genius idea to add a -20dB pad on each channel of their U-Phoria HD interfaces. Everybody else seems to have enough gain and enough headroom for anything I threw at them.
But really, I can't say that I've come across an interface that I haven't liked in the sub-$300 range so far. Except the Lexicon Alpha. That thing was awful. But it was also a 10+ year old product when I reviewed it. It just shows how far things have come. There's hardly such a thing as a bad choice in the interface market these days. They're all pretty damned good! So if you're looking to upgrade, my advice would to be to upgrade for features and not for sound quality.