Although the iDA X100 and iDA X200 look very similar apart from the screen, you'll probably find that the internals of the iDA X100 will be better due the higher price of the head unit. This will mean that it works faster for example and may even be more powerful. I haven't compared the spec of both of them, but I'm guessing this would be the case.
£90 delivered for Skyliner34's iDA X200 is a steal though, unless you're particularly bothered about LCD screen etc.
Worth mentioning that neither head unit is compatible with the iPhone.
As for which iPod is best, I would buy one with a capacity that suits your needs. All of the user interfaces are very similar and easy to use. Don't get a shuffle though (No screen = pants).