So many variables and such a question with proposed budget will easily get 100 different suggestions from as many different posters.
1-Whether you have Firewire 800 or 400 on your computer. Some newer MacBookPro’s only now have Lightening or Thunderbolt.
2-USB interfaces have come a long way but if you are running a number of other (for instance) Ableton Live peripherals via USB or even other Native Instruments similar Sample/Launchpad type devices…then an USB Audio Interface may not be greatest option due to fact that audio Interfaces using true high speed USB 2.0 bandwidth, and do so very efficiently with multiple audio and midi tracks, tends to be somewhat USB bandwidth hungry and Fire Wire may be better.
3-I do not use VSTi’s at all, having vast hardware synth set up AND hardware effects racks–takes quite a load off any computer and audio interface, esp. effects.
Earlier this year I went with Focusrite Scarlett 18i20 rack unit USB audio interface and I happen to use Reaper for my DAW with the SWS Extensions. Have had 8+ hardware synth audio inputs recording at once, sister midi tracks for most if not all (except the A4…), plus for the $499., you have ADAT expandability, SPDIF, and Word Clock.
4-For same amount of cash you can get Focusrite Saffire 40 Fire Wire, which my Scarlett 18i20 USB is actually the exact same preamps, features.
5-You could easily spend ALL of $1000. on something by RME but even Behringer just released a new dual Fire Wire/USB capable Audio Interface for only $250. that’s essentially a copy of my 18i20, with addition of Behringer’s newly acquired company, ‘Midas’, and that interface has MIDAS Preamps. That would leave you coin for a mid range price set of decent monitors.
6-suggest reading reviews on various interfaces on SOS, home recording dot com, audiofanzine, KVR, evn gearslutz, because nothing beats knowledge to be gained/shared in any of those places.
7-Lastly, although there’s so many other variables to one’s personal needs; I wanted to have level knobs for majority of channels, including my monitor outputs, a power switch, and dual headphones out–all on the front surface of rack.
also wanted audio interface to be able to work in standalone mixer/summing mixer, of which I happily got all my desires with the 18i20.
Whatever you project as your ‘minimum requirements’, learned it’s pragmatic to simply double those numbers as to have versatile routing options and expansion options. Why keep buying another interface for more channels when do it right first time? I can even use my iPad along with camera connection kit with my 18i20 if wanted…bang for your buck!