Well, MySpace apps are relatively new, so most ppl aren't ignored/blocking them yet. Also, MySpacers view 4 times more pages per visit (on average) over Facebook users. scriptamp.com discusses a few things on myspace apps, and some other neat tips and tricks to monetize on myspace with apps.
Consider the fact that Myspace is currently conspiring to BUY iLike, the most popular music app on FACEBOOK! Myspace apps definitely make money, but the site has seen a decline in it's growth through the past year, while Facebook has grown by over 150 MILLION active users in the past year. Yes, about a year ago, Facebook had around 100 million active users. Now, they have just recently hit 250 million active users. Facebook rules, forget Myspace.... Even Myspace wants Facebook apps!
I would say facebook would give you a better return. I've seen a shift from myspace to facebook and I know you can still ride that wave. I would start with facebook and than add myspace. A better answer would be why not do both and compare the two, than eliminate the one with the least results. Have fun. -JC