Are the A-sides and B-sides of the wonder boards really balanced?
From what my friends and I can ascertain, the board sides are not designed to be strictly balanced per se, but are instead designed to work best with different play strategies, or at least different skill levels.

In my experience, the "A" sides provide fewer opportunities for specialization than their "B" side couterparts. For example, if a player is playing with Rhodes, but fails to take advantage of the early military points and never plays a military card, that player would have probably been better off with side "A", which provides a few extra victory points and more closely matches the costs and benefits of the other wonders.

That said, it doesn't take much more than a basic working knowledge of the game to make the "B" sides more than pay for themselves, especially with a wonder as unbalanced as Rhodes (other wonders' "B" sides aren't quite as obviously better). In our games, even novice gamers rarely select side "A" after their first couple run-throughs, and when they do, it is rarely a winning strategy.