I've owned the 2120 and also own a SGX2000.
The 2120 has seamless changes, which kick ass, but the seamless setting eats up processor usage. The seamless stuff can really kick ass for things such as allowing a delay to fade away after switching to a non-delayed setting.
The 2112 might also have the seamless changes. Also, there used to be an upgrade available for the 2112 to turn it into a 2120.
Other cool things about the 2112/2120 included intelligent pitch shifting and especially the ability to split distortions left and right. Thus, you could send the SS distortion to a left speaker and the tube distortion to the right to obtain a ballsy full sound.
However, none of these cool features changed the fact that the 2120 had a weak basic tone. After owning it for years, I could never get a good raw tone out of it. Even when used for effects with another preamp, it seemed to digitally color the tone.
Whoop, if ya want tons of effects, the 2112 is decent. If you want good tone, it's a tradeoff. You've already used it and seem to like it. In fact, the 2112 is more similar to the 2101 than it is to the 2120. If you can get it for cheap, it may be worth it.
Alternate recommendations?
Mesa/Boogie TriAxis with Lexicon or TC Electronics effects:
Footwitchable between settings via MIDI. Killer tones and pristine effects. Arguably, there is nothing better.
SansAmp PSA-1 with similar effects:
Also footswitchable MIDI control. Great tones. Cheaper than the TriAxis, but the TriAxis has the best tones.
However, both of these setups may be more expensive than the 2112.
If you want really cheap multi effects,
the SGX2000's should be pretty cheap. However, they drastically digitize your tone. It's not terrible, in fact it can be cool, but I prefer a rawer tone.