Okay, DO NOT do what I think is called linear triple science- basically where you take one exam for each subject at the end of the two GCSE years. It's hellish, since everything rides on how well you do in just the one exam for each subject, and the pressure is much more immense than modular triple science.
If you're doing modular, then I wholeheartedly endorse it, especially if you plan on doing science-y A-levels or even degree. It is a step up from double, but it is worth it because then the step up from GCSE to A-level, although still quite large, isn't as daunting.
Of course, if you're going to do worse in triple than in double by a large amount, then perhaps it is best to do double. Personally, I was predicted two A*'s and a A at triple, but just ended up with straight A's. I know this is still okay, but I think I'd prefer to have two A*'s instead. Then again, doing physics A-level is easier for us girls that did triple than for the guys who only did double, since we did seem to cover a fair portion more.
Meh, it all depends. Sorry if that wasn't helpful. :P