Choosing the right audio-interface

There are very many audio-interfaces on the market.
How to choose the right one?

RME Fireface UC, a good USB audio interface with many options

Here are some important considerations:

• What's your budget?
• Pci, Pcie, Firewire or USB? Built in or external box?
  Pci and Pcie is Pc only. Mac goes with Firewire. For lowest latency: Pcie, Pci, Firwire, Usb (fastest first). For easy portability: Firewire and Usb
• How many inputs (converters) do you need?
• How many will you max need in the future?
  This is how expandable the unit is. Adat? SPDIF?
• Do you need onboard preamps? How many?
  The quality of the preamps is a main factor. Are you really gonna use them for studio-grade work, then they better be good. However, if you're new or on a tight budget, some ok built in pres does the job well.
• How many audio out?
  Just 2 out to your speakers/headphones, or do you need more?
• Flexibility with more headphones/ monitors?
• Converters: What kind of quality are you aiming for?
  Most interfaces have decent AD and DA converters, but if your aiming at studio-quality, you might go for pro ones like RME, Lynx, Apogee.
• Midi in/out is typical standard for every interface, but be sure it's included.
• Do you need to operate on very low latency? (Good for monitoring software- instruments/effects)
• DSP (fx, eq's and stuff).. nice, but how much do you need it?
  A DSP-mixer is another story, be sure a flexible DSP-mixer is included.
• Look and feel: Is it gonna be at your desktop? In your rack? Just some breakoutcables from your Pc? Fancy? Solid? Portable?