True, she has more upside based on her size/athleticism combo, but what are the chances she achieves it? Fowles has been the league four years (granted she was injured a lot the first couple) now to Charles' two, and their numbers are already comparable:
Fowles: 20.0 ppg, 10.2 rpg, 0.6 apg, 1.2 spg, 2.0 bpg, 59.1% FG, 76.6% FT
Charles: 17.6 ppg, 11.0 rpg, 1.7 apg, 0.8 spg, 1.7 bpg, 46.8% FG, 68.7% FT
And Charles put up those numbers on a team that won 7 more games.
There's also the stark contrast between their sophomore seasons:
Charles: 17.6 ppg, 11.0 rpg, 1.7 apg, 0.8 spg, 1.7 bpg, 46.8% FG, 68.7% FT
Fowles: 11.3 ppg, 7.8 rpg, 0.8 apg, 0.9 spg, 1.5 bpg, 59.9% FG, 64.6% FT (missed 10 games because of injury)
Conventional wisdom says that Charles has a lot more developing to do between the two. That doesn't mean that Fowles - who's still just 26 - won't develop, too, but between the two, Charles likely has more growth as a player ahead of her.
Here is how I would compare the two going forward:
Charles is a good defender, but Fowles is the best post defender in the world BY FAR (excluding Griner for now since we haven't seen her against pros). She's also an extremely efficient offensive player who's developed a respectable post game, and now takes advantage of her free throws as well. Charles, though, is the more dynamic player - she has the most polished low post game (aside from maybe a healthy Parker), a good 15-footer that should get better, and she's more fluid off the bounce than most centers.
I see Fowles being more a Dwight Howard-lite (minus the character issues); she can be the best player on a championship team, but you need a perimeter player who can score most of your crunch time minutes. Charles, on the other hand, could be the best player and best scorer on a championship team.
I think these two will always be extremely close - especially since Fowles' defense is so freakin' good - and flip-flop from season to season who is the better player, but ultimately I think Charles will be the more dynamic all-around player.