I have a Focusrite 2i2 (the small one) and it works great. I've needed customer support (repairs actually) and that was a positive experience (had it back really quickly).
Since a few days I've now got a Steinberg UR824 interface. It is actually based on Yamaha technology (who now own Steinberg) and it sounds EXCELLENT. Might still be a bit outside of your budget though, but certainly cheaper than the Roland. One of the things I like about the UR824 is the built-in DSP, which has an excellent sounding Yamaha reverb (global, not per channel) and DSP-based channel strips allowing for a compressor and EQ.
There's also the Mackie Blackbird, which from a technology standpoint might be similar to the Focusrite, except that it uses Mackie Onyx pre-amps. I've never been a Mackie guy though.
Most interfaces advertise quite a lot of channels (38 in total for the 18i20?), with the majority of them being digital. For example, the Steinberg UR824 has 48 channels (at 48 KHz); 8 analog in, 8 analog out, 16 digital in (ADAT) and 16 digital (ADAT) out. Practically speaking, I'd call the UR824 a 8 channel interface, or perhaps 16 (8 in, 8 out). The 18i20 has 8 analog inputs and 8 analog outputs (10 if you include the two monitor outputs).
Here's a hires image of the read of the 18i20: I had to do some searching to find one.
If I look at the Roland, it has 16 inputs (12 on XLR/jack combi inputs and 4 jacks) and 10 outputs (8 jacks and two monitor outputs on XLRs). Yes, it's more expensive, but also seems to offer more than the others (which have half that number of analog inputs).
And of course there's the MOTU 828mk3 Hybrid (Firewire + USB) which also offers a DSP mixer and 8 inputs/8 outputs + digital stuff.
I needed to sync my new audio interface to the Word Clock of a different interface (RME), so the two options (given my budget) was the MOTU and the Steinberg. I got the Steinberg because it sounded better to me, even though the MOTU probably had more features.
All these audio interfaces pretty much have two headphone outputs (can be assigned to different mixes), a DSP mixer (so you can use it as a mixer even without a computer connected) and some on-board effects. Some have MIDI, some don't so it that is important for you, check it!
Until recently the Steinberg UR824 could only record at up to 96 KHz (probably plenty), but there is a firmware update that extends that to 192 KHz; the box of my unit still said 96 KHz (and it had firmware v1.0), but after updating it to firmware v2.0 it will now happily record at up to 192 KHz.
These interfaces all seem to have only two channels (the first two) that can be switched to Hi-Z mode (for electric guitars). Also, while some contain a DSP mixer (Focusrite, Steinberg), you need to set it up with a computer attached (balanced between channels, effects, etc). - after that they'll happily work without a computer attached. I doubt any of these interfaces will work completely when powered via USB (if that is possible at all); perhaps you might be able to use something like the first 2 channels, possibly without phantom power. A separate power supply is pretty much a given.
A bit of a braindump, I hope you're okay with that. The Roland looks quite good (specs) and I've seen a lot of positive remarks about the smaller models, so the sound quality of this one is probably going to be great as well (but it is outside your budget, as you said). The Steinberg (Yamaha) might be an option and the Focusrite will probably work great too.
There's also the M-Audio FastTrack Ultra 8R, but I'd steer clear of that one. The hardware worked fine (I had the FastTrack Ultra desktop version), but the drivers were buggy as hell. Right now you can't even find it anymore on the M-Audio website, but I still see them for sale sometimes.
Presonus probably also has some interfaces in your requirements/budget bracket. They're a decent brand and the quality will probably be fine, but it is my impression it takes them quite a while to update drivers. That said, I have no experience with them and did consider getting one of their digital mixers (StudioLive) to hook all my gear up to.
Btw. some interfaces can be used from an ipad. You could record on the ipad (instead of the Mac) and perhaps control them from that. I know the Steinberg recently got ipad functionality and the Roland probably does too, as might some of the others.