Hey man, thats a choice that could almost be dictated by how much money you wish to spend! If money is a concern, then the 240SX would be the better choice. I am going to assume that money is not a concern here.
I would go with the FD for a few reasons
1. Weight distrubution is almost Perfect at stock.
2. i know how to build rotarys, and IMO, it is more simple than building a piston engine
3. You put a full exhuast (turbo back) on these turbo rotarys, and it is common to gain 50 Horsepower to the wheels.
4. Rotarys respond well to mods...especially turbo rotarys (see above)
5. You can fit some massive tires under those FD's wheel wells.
6. With a mildy ported single turbo rotary with good engine management, you can see 500 Horsepower easily. (although the enigne will probably last 30,000 miles <img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/tounge.gif" border="0" valign="absmiddle" alt='
'> )
7. The car is freaking beautifull in stock form.
Those are just a few reasons why i like it!
All in all though, i love my S13, and love the memories i have made in it. And its been real reliable. If i had lots of money, i would be tempted to build both!
Tyler