I think Kevin Martin will have overall value, but Beal will probably have more eye catching stat lines. Martin is usually under rated because he doesn't have to score 30 points, or pull down 15 rebounds to have a lot of value. His %'s, super high 3PTM, nice steals and relatively low TO's for his position are what make him valueable, even if at a quick clance, 15/2.3/1.3 doesn't look very good.
Beal has good upside, and might finish with more PPG and similar steals. But I see no way that he stays competitive in 3PTM or %'s. He's averaging less than 40% from the field on the year right now.
So, I would not take a few more points, and maybe assists for downgrades in %'s, 3PTM and TO.
I do like Beal though. I think next year he will be a must own player in nearly all formats. But he's definitely not more valuable than Kevin Martin, at least right now.