I`d go for external, and firewire. the NI thing is usb2. Which I just wouldn`t trust.

There`s this
http://www.dv247.com/invt/25936/

this
http://www.dv247.com/invt/19660/ (I use this, it`s rock solid, has 24/96 output, and you get live lite with it, meaning you can upgrrade to ableton 6 for about 150 quid), and has the a/b outputs so you can have monitor or main mix out in the headphones.

http://www.dv247.com/invt/35953/ echo stuff is pretty stable

http://www.dv247.com/invt/33078/

Unless you want loads of outputs, I would say you don`t need to spend over 200 quid. But definitely go fireiwre.

I`d recommend the behringer controller over the UC-33.
It feels nicer, and the faders are great. Although the all knob version is good too.

If you have a slightly higher budget then this controller is simply raw sex
http://www.nusystems.co.uk/product/CME.Bitstream.3X
You can order them from the states for about 200 due to the exchange rate, but they are 300 over here