Bethany is much better than her jerk of a twin Carver. If you leave him behind, he joins the Templars even though you are a mage. That's the biggest middle finger I think any companion has ever given the protagonist in any Bioware game.
I'm doing a mage playthrough, and Carver is a particularly hard character to like, though much of this has to do with the fact that he starts off with a high enough rivalry that it's almost impossible to gain favor with him unless you take him everywhere and do pretty much nothing but pro-templar stuff, which, as a mage, I cannot do. At best, I've been able to get low rivalry, but never really could get him into the neutral zone before having him go off with the Grey Wardens. I will say, at the very least, that Varric and Isabela giving him a hard time makes up for it considerably, and I enjoy Varric's apparent disdain for him.
As for joining the Templars, it seems that part of that had to do with the fact that he was denied admittance into the City Guard, which you only find out if you have both him and Aveline in your group. Apparently, Aveline doesn't approve of his character either, since he's an insufferable git.