Actually, for the Redcliff quest line doing the Mage Tower first leads to the "best" ending of that quest line. If you don't finish the tower, you aren't even given the dialogue options.
The game has lots of little paths that branch and some small choices cause branches you never might never know notice till you play again. Personally I love it.
The only thing that prevents Dragon Age from knocking Baldur's Gate II off the Throne of Bhaal is the lack of multi-player. Otherwise, I'm 30+ hours in and still loving it.