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If you're looking to make some serious power and have the money to do so, the 240SX is an awesome platform. The Prelude will be a more dependable DD though. My 240SX was never what I would call reliable.
The 240SX came with the KA24DE which was DOHC, at least the S14 did. I believe the S13 came with the KA24E which was SOHC. I've never heard of a VVT head swap on the SR motors.
Early S13's came with KA24E SOHC engines, later S13's came with KA24DE DOHC engines. The change came around 91.
The SR16 was a VVT engine, AFAIK.
Buy a 240. It's rear drive and accepts much better engines then the Prelude ever will.