I would choose beauty because it comes with a lot of benefits. People prefer "beautiful" faces. Brawn is nice in essence that you're fit, but I don't want to exercise and pump iron compulsively (not everyone does that). You can be fit without being super-muscled anyway. Plus, then I might have beauty and my brain (which is not saying much).
While in many cases beauty and brawn go together (fitness) if there has to be a separation between the two the ability to get things done will always trump the ability to look pretty; those who are seen as useless are not particularly sought after except for breeding purposes meanwhile those who have utility are sought after for many purposes including breeding.
Just being beautiful doesn't contribute to anything at all-except for vanity. Sure, its great to put on some makeup and do your hair, take care of your general appearance (brawn is also taking care of yourself), etc. However, your body can still be fit and "beautiful" while still being able to contribute to activities and life vs. Being a couch potato and having health problems.