The E-MU "Proteus" series hardware are basically all identical, except for the ROM samples and programs they come with. So essentially, if you are going to use the built-in sounds only, the choice is very important. If you however, are going to create your own sounds, the choice becomes a little less important.
I am not a using-the-programs-it-came-with kind of a person myself, so I tend to choose based on capabilities rather than samples and programs. I have to say though that, if you are going to edit on any of these modules, it's very tedious to do on the unit itself. It is possible of course, but as I said, it can be frustrating.
Capability wise, the units are very capable, but personally I would choose one of the more advanced (read later) modules, since they will have an edge over the older ones in terms of what it can do. This choice is, like I said, because I am interest in making sounds.
The samples on these modules are really very similar. They are of course different, but what I mean is that they are aimed at modern (for THEIR time) music. Progressively, raw material is becoming less important because machines or software can render them incomparable to their origin.
With Note Expression (VST3.5), individual notes contain their own set of controller data when played polyphonically.
VST Expression, Steinberg VST
Cakewalk Sonar X3
now supports VST3!