IMO, it has to do with desired end results.
1)Are you wanting to race this car, show the car, or just have something fast on the street?
2)If racing is your goal, what type of racing, drag, drift, autocross, etc. The S30 and S13/14/15 were better meant for some things than others ie: If your objective is to drift, get the S13/14.
3)If you're showing the car then its about showing it and should be a self preference. (my $0.02 : originality is always best, and I'm sure the s13/14 - S30 ratio is like 8:1
4)If you just want something fast, and don't have any other objective, then you're looking at cost and comfort. Both swaps, (depending on the S30 chassis condition) are relatively close in $ amount, but there is more to be swapped into an S30 chassis to better handle the hp/tq you may be running (ie: bigger brakes, suspension, LSD although not entirely a necessity). So if going the S30 route, you may be looking at dishing out more.
Also forgot to mention that you would need to set up the 260z to run fuel injection, like using a 280z fuel tank or fuel cell, fuel pump, etc, whereas the s13/14 is already set up for that.
Ultimately do your research, and do it thoroughly, and write it down, its easier when you see EVERYTHING on paper (or screen if you're typing) in terms of what kind of work you have to do for the swap. There are many sites that have comprehensive build threads and swap info on the 240sx as well, just can't think of them off hte top of my head other than some local ones.