We’ve been to both, and both are worth seeing. I concur with everyone that Leu combines well with other Winter Park area sights, including the Winter Park Scenic Boat Tour and the Morse Museum (largest collection of Tiffany pieces in the world). It’s also a shorter drive from the WDW area.
Bok Tower is really unique, though. The gardens are great, and were designed by Frederick Law Olmstead (designer of NY’s Central Park). The tower is beautiful. Carillon music is also unusual, at least outside of Belgium and the Low Countries. It’s worth trying to be there for the 1 pm or 3 pm daily concerts. The cafe at Bok Tower was better than we expected last time we were there. Bok Tower is very near Legoland, but doesn’t really pair super well because Legoland is typically going to eat up most of a day.
Really, it’s hard to go wrong with either one. If you do go to Bok Tower (or Legoland), you may want to stretch your legs on the way back at Davidson of Dundee, a semi-famous citrus stand and candy factory. It’s no Wall Drug, but worth a stop. You really can’t be sure whether you want a gator-shaped citrus jelly until you’ve seen it with your own eyes.