I have a 2101, a 2112 (upgraded to a 2120), a Pod 2.0 and a Pod XT. I have also owned a MP-1.
Here is my opinion on them all:
2101:
+ Best distortion of them all. The heavy distortion is done in two stages and each is adjustable. Very good effects, especially when they have the optional second S-disc processor board. If you want freaky sounds and sick distortion this is the one to go with.
- Only a so-so clean sound. The effects are very complex to program and there is no real patch editor available and never will be. The Control one is a must. Crappy through headphones.
2112/2120:
+ Good distortion, very nice clean sound. Nice options with 2 preamp paths, you can use a CC pedal to smoothly go from clean to distorted and anywhere in the middle. You can vary effect parameters at the same time. An excellent PC patch editor is available. Excellent display that shows a lot of information.
- Distortion not as good as the 2101. Effects are not as configurable as the 2101 (though they are easier to program - even without the patch editor). The Control one is a must. Crappy through headphones.
MP1:
+ Does the marshall sound well, clean is pretty good. Very easy to use.
- Pretty limited range of sounds and only one effect (Chorus) IIRC. Crappy through headphones.
Pod 2.0:
+ Cheap. Easy to program, with or without PC editor. Lots of useful sounds.
- Limited effects. IMHO not really a contended now that the Pod XT is out. Sounds good through headphones.
Pod XT (would also apply for XT Pro):
+ Sounds great and has a lot of sound shaping options to dial things in. The models do sound a lot like the real things. Has USB output for direct to PC recording. Sounds good through headphones. Easy to program, with or without PC editor. The best "feel" of any of the preamps I have listed.
- Can be hard to dial in - but so worth it! Effects can be controlled via CC but with a lot less flexibility than the 2120. The Pedalboard is a must, and they are pricey.
I will duck for cover after this statement but I would go with the Pod XT all day long. It sounds great and when used with the Floorboard controller you really get the best of the rack and pedal worlds combined. Basically take a look at the manuals and see of the modeled amps and effects (including model packs if needed) float your boat. If they do then you are all set. If there are a few essential effects missing then you can still use it with a pedal or two. If a bunch are missing then maybe it's not a good fit for you.
Let us know what you decide.