Ok, lets see if we can clear some of this up.

Rollers: Same set can be used for MTB and Road bike, almost
all of the good rollers are adjustable for wheelbase. Tire size
makes no difference (at least I don't think it does for the
rollers to function). I've not used rollers with MTB,
but I'm guessing that the knobbies due to lower
rolling resistance will give you more difficult workout,
also might be kind of noisy. You don't need different
roller for the MTB tires.

Trainer: Bike is secured by a clamping device that screws onto
rear quick release (at least the ones I've seen/used), elevating
rear tire off the floor. There is a roller attached to a
flywheel and wind, magnetic, or fluid, resistance unit that the rear
wheel turns. You can change gears, and most have resistance
settings. The fluid units are variable resistance and have no
preset settings, they are also much quieter than either the
wind or magnetic units.
With the trainer there is no learning curve, bolt the bike in
and away you go (well not literally).
With Rollers there is a learning curve to initially get the hang
of riding rollers. Eventually your balance gets good enough
that you can ride no handed (I still can't do this on rollers),
or so I'm told.
Rollers take a degree of concentration, stationary trainers
require none.
Hope this helps out. Do a search on trainer or rollers
here there have been a few threads covering them.
Nathank, if you were not in Munchen I'd say come over
and you could try them both out. That said, you might
check with LBS about testing. In US Kreitler are far and
above the number one brand. I'm betting they would
cost a fortune in Europe, but Tacx makes a nice roller
(and trainers for that matter) and they're a dutch
company so should be cheaper than here.

Marty