Hello again Stefaan,
As mentioned before, whatever firewire interface you choose make sure it has a built in pre amp. That boosts the audio signal to acceptable levels without compromising quality. The two devices mentioned have both XLR and 1/4" inputs with 1/4" being the plug like one you would see on a guitar cable. If you look at the product shots for the MOTU 8Pre posted below on the inputs there's a hole in the middle so you can plug in a guitar, etc. that has a 1/4" plug. This type of input is common on professional audio interfaces.
Click on the image to see a larger view.
Number of simultaneous inputs may not make a difference at this time however they may come in handy in the future. For instance if you are recording a podcast with a mic for each person in an interview you can connect the additional mics and have each as their own individual track in the software that you can edit individually without compromising the other audio tracks. Firewire interfaces can also be daisy-chained to allow for additional inputs. Of course you are left to the mercy of subsequent limitations from your hardware in the respect to real time processing of multiple tracks simultaneously. I bought one of the cheapo-s at first but it didn't have a preamp and as a result the audio was less than desirable. I got the MOTU 8Pre after researching and testing both the MOTU and the Presonus. There's pros and cons to both interfaces and if you're interested in either and do your hands on research you'll discover for yourself. Needless to say I went with the MOTU and I am happy with it.