James brought up some good points, but let me share this bit o'pinion with yew:
The 280Zs tend to be newer and have less problems (read: rust, wear, etc) then the 240Zs do. If I could, I would have built up a 280Z in heartbeat, but smog laws here prevent extreme modifications. The 280Zs, at the very least, feel more solid then the 240Z. The door mechanisms are better (IMO), and so are the window mechanisms, etc. I have taken apart both doors, and I liked the 280Z door much better. After going through several 280Zs, I have to say that the fit and finish is better than the 240Z IMO.
Weight can be an issue, but I don't think a 280Z would have any problem moving out with a V8 under the hood. The 280Z is quieter, more comfortable (although some may argue this), and they feel more solid at speed, possibly due to the added weight? The 'frame rails' on a 280 extend back farther then they do on a 240Z, although ultimately with big horsepower you'll need some additional stiffening. Check out Pete's site for great info.
As far as the styling between a 280 and a 240, well, the bumpers are cleaner on the 240Z, but that is really about it---they look VERY similar in every other way (don't nit-pick, guys)
Whatever you do, buy a rust-free car. I didn't, and it took me years to find the 'right' car that I can now cut up.
Davy