I would think that if your main light source is going to be the flash, then flash or sunlight WB setting would be give you the most normal looking colors. Auto WB should be adequate, I would think. Of course if you shoot in RAW, you don't have to worry about that.
I don't recall if the 10D has E-TTL II or just E-TTL I. The 580 is more powerful but I think it may have newer technology than your 10D can use. If so, the 550 might be a good choice. Also, I've been hearing good things about the Sigma 500 Super. Supposed to be fully compatible with all digital EOS cameras and is half the price of a 580 ex.