hi and welcome to the forum

first of all you need to know what you are going to do with it!!

i.e. just a sunday morning toy or a full on street wepon/ track days etc

i found my self in the same position a few months back and ended up with a mc28 se

for just a sunday toy you can go for either 21 or 28 they are both pretty quick in standard form and draw loads of attention .
i personally like the 21 body shape with nice sharp edges though i like the pro arm on the 28.

if you want the ultimate street/ track tool then you have to go for a 21 as they are alot more easy and cheaper to get full power
21`s are 45bhp standard and 60-70bhp is only a 1hr job no cost
28`s are 40bhp standard and 55-60bhp will cost you a grand if you have no hrc card

hope this helps