The weight also has a bearing on your track-related wear items: brake pads, rotors, tires. At low mileage you probably won't get into things like bushings and wheel bearings, but they're eventually going to go.
Porsche builds its sports cars to have enough stop and turn to survive repeated hot laps. BMW doesn't. You're going to have to - at the very least - switch pads and fluid in the M3 to match a stock Cayman. And in Florida heat at Sebring, these wear items don't last. And the M3 is going to pound through tires at a much faster rate than the Porsche.
If going to the track is that important, get the car that will last longer, go harder, be more reliable, faster, cheaper to run, and have better dealer support. We all know that's the Cayman.
For me personally, I used to be a huge track guy, more than 15 events every year. I went through the expensive process of converting an E46 M3 for track use, and after 23k miles and countless thousands spent in keeping the car safe, reliable, and competent on the track, I gave up and opted for a Porsche. There's no question in my mind a Porsche is a better choice for frequent track work. Add in the superior dealer network (IMO), and the choice is made for me.
That said, I prefer BMWs for the street. I like the lower profile, greater comfort, whiz-bang gadgets, and lower price point. That's why I'm choosing an E92 M3 over a Cayman variant or base Carrera. For the track, I drive Skip Barber MX5s.