I have both so I just bounce back and forth between the two! 
But Halo Reach Beta I find more fun and you can get right into it, the Starcraft 2 beta takes some getting use to and can be tense, even if you win in the end.
But Halo Reach Beta I find more fun and you can get right into it, the Starcraft 2 beta takes some getting use to and can be tense, even if you win in the end.