Barfield is blocked by Loretta. I think this is Loretta's last year in his contract, but don't know for sure. Barfield struggled early in AAA, but has hit well the last month or so. He still needs to work on his plate discipline (strikes out too much), but, should be a solid 2B. I wouldn't be surprised to see him starting for SD in 2006, possibly with Loretta at 3B.
As for Burke, the issue is Biggio. He is a fan favorite who is closing in on 3000 hits. He's playing 2B, which is Burke's natural position. While Burke has been playing a little OF, he is not a CF. As such, he really can only play LF (presuming Berkman in right and Tavares in CF). Yet, the Astros have Lane as well, which cuts into his PT. They have gotten around that a little by playing Berkman at 1B during Bagwell's injury, but come 2006, if Bagwell returns, that won't be an option. As such, while I think Burke is ready for a full-time job now, he needs for Biggio to go (so he can move to 2B) or for Lane to get traded to open up a full-time job.
My opinion on skills - Barfield will have a little more power and Burke a little more speed. If Barfield can cut down on his Ks, he could have a better average, but right now I'd go with Burke as having better potential in that category.