The internal routing and conversion is the same in each unit. From an analog perspective you lose two mic preamps, two line inputs; and lose two balanced outputs. From the digital side you lose a Toslink (8 channel ADAT) input. Also, two of the line inputs on the 400 are combo inputs which by definition is not necessarily bad. You can mic a kit with a stereo pair if you need to but four would be more common and you would of course lack preamps for that. The Fireface preamps are quite usable by the way.

So if i hook up the ADAT with an 8 mic unit would I have 10 usuable mics to record drums with the 400?

Correct. Something on the lower end of things would be the Presonus Digimax D8 or a used Mackie Onyx 800R. On the entry pro level is the Octamic D. On the high end you have the Micstacy, various offerings from Millennia, Grace etc.

Ok i haven't used either one, so sound quality judgement is out. Connectivity wise, i'd go 800. i love the sound of two 81's in a coincedent pair over a kit. Some projects they're perfect alone, others, it's the 'salt and pepper' to the drums.
at the studio we usually have 14 drum channels, 2overhead,2room,kick in/out, snare top/bottom, top each tom, hi hat, ride. There's usually a guitar, bass, vocal, going scratch at the same time.
I know it costs $, but if i were going to invest personally, i'd get 800 in this case, if you never use all of the channels, you still can, if you want more than the 400 has, you need to buy a whole new device.
Depends on your current/future needs, and your wiring/patch scheme.

Wel thanks for all the advise guys, I am gonna go for the 800 it just makes more sense. If im gonna layout that much might as well wait a week or so and get the 800.