I wouldn't start from scratch- I would buy my S14 that's for sale and drop an SR in it. replace the 8 yr old coilovers, and you'll be well on your way to a competitive car. (I bought an EP RX7 that is/was quite a bit faster for the price it would take me to put an engine and suspension into the 240SX, and EP is quite strong here with national-level competition at every single race. STU is lacking in tough competition, so it doesn't make a ton of sense to me to spend a bucket of money to make it faster with little competition to run against.)

The S14 chassis is a touch bigger and heavier, but it's also a stiffer chassis and has better geometry in the rear subframe so you can get better corner exits. With the 5-lug hubs, you also get better wheel bearings that last a lot longer than the S13.