I'd probably go the rotary, specifically because of the weight (or relative lack of weight) of the vehicle. The rotaries (and please bear in mind this is a relative thing) in general produce more power but less torque than a comparable piston engine, which should suit a lighter car and a race-car at that. Plus, there's something quite enjoyable about the powerband of a rotary, even a turbo rotary.

If you are talking relatively standard engines, and making a complete guess as to how many hours you might cover in a year of competition, it's hard to knock the reliability of either one I suspect - you'd be running for a good few seasons before even having to think about any engine freshening up.