I'd definitely stick with Canon. Top-notch AF speed, even on the dinky consumer level lenses, and the best lineup of telephoto lenses on the planet. Due to selling zillions of them since Canon has owned the sports market for the past 20+ years, they're also surprisingly affordable, at least compared to other brands. I'd go for a new or used 30D or a new 40D. Then get an EF-S 10-22 for skateboarding stuff, and then maybe a 70-200 f/4L (non IS) to cover snowboarding (assuming you'd be further away). I'm not sure how you're planning to shoot, but if you're going to be directly underneath both, get sure to consider the Sigma 10mm f/2.8 fisheye lens.