All the Eventide processors are basicly the same. The older and obsolete GTR4000 was the same as the DSP4000 but with a few less features and the presets were targeted more for guitar players. You could buy PCMCIA cards that had presets from the other models to get the presets for the other specific models. The more current Eventide processors include most of, and the best of, from the presets of all the previous versions along with additional and updated preset versions.

I never used one, but which model (old or new) would be the best for a project studio / guitar player?

What would be best is to have the top of the line current model, the H8000FW. It has the best of everything they have ever made.

If your budget and needs are more meager, then a used GTR4000 still makes a wonderful guitar effects processor within it's limitations.

You are most likely referring to Steve Vai and the H3000s. The H3000s was touted as his model, and contained a slew of presets he used/made.