[timytamy]

I'm going throw a completley biased opinion in and say Metric.

We've been standardised on metric out of neceesity, but even then we've never had great dificulty with intrerfacing with AM/VEX etc. It's true you need a second set of tools for many things, but I've found that more than enough socket, spanner and allen key sets often come with both Metric and SAE sizes anyway.

Don't stress too much about mixing bolts, in fact, stripping out a screw head can be a very good leason in using the right tool - not just the one that fits! That being said, you don't want that to happen at a competition!