I use TrueCrypt to protect my laptop. No chances in speed that I've noticed and as far as I understand it, it's quite secure. 30+ randomly generated password gets me in and it's a lot easier than you think to memorize and type quickly. I always feel like I must have made a mistake but can't remember the last time my fingers got it wrong. To me, it's kind of a no brainer. 3 extra seconds before boot is the only thing it takes to make sure my stolen laptop couldn't lead to stolen credit card numbers, accounts, business docs etc.

I've got no experience with Bitlocker, but I'm a huge fan of TrueCrypt.