Is this "of people who have a reasonable shot?"
most of the population is precluded from the NBA based on height. but height matters way less in baseball than basketball, so the pool of competitors is waaaay bigger for baseball. You have to consider the fact too that baseball players generally make much more money, so a lot of on the fence athletes will swing that way, increasing the talent pool even more. Additionally, baseball players stay viable for longer, due to the less strenuous nature of the game.
If we count major leagues only (discounting euroball, d-league, aba, and the minor leagues) and narrow it down to athlete X who is equally talented in basketball and baseball, it's tough, but I'd say it's harder to make it in baseball. Unless generic athlete X is going to be a star, remember that he can make 5 times as much money by being a baseball roleplayer than a basketball roleplayer, so he's going against all other generic athlete x's.
