I wouldn't consider a 750 over a 620 - for the power difference the price difference is not justifiable (of same year, cond., etc...) And as far as I know the 750 and 620 are the same physical size - correct? I have a 2003 620 and love it - I wouldn't mind having a 900, but then again I am a fairly laid back rider and am not concerned with the hp's most of the time so I wouldn't really benefit from that. So that just depends on what you are capable of handling, or are interested in.
As for the price on the 2001 Monster - try Kelly Blue Book - they seem to be close on most vehicles I have looked up. Also check cycletrader, the classifieds here, places like that to see what the bikes are actually going for.
Good luck - I say go for the 620, but I obviously am biased ;D - what can I say, I LOVE my Monster.
Yes, the 750 and 620 are the same size. Really all the Monsters are basically the same size, but there is a lower cut "city" seat on some 900s and 620s. Of course the S4 has entirely different bars and definitely sits slightly taller. I've got the 620 and would never trade it for a 750. As has been discussed here before, there's barely a 2hp difference between them. The 900 definitely has more oomph, but I really like my 620 for a do everything ride. Not too much, not too little, just right.