I havn't used a bowl to bake but I've read instructions on lots of cakes that have, I'd defo go for the bowl, carving a nice round shape will just be hassle and mess!
I've baked in bowls lots of times. It works just fine. The bowls have a small flat area at the bottom, so to get a perfect round you'll still have to carve a little bit, or (what I would do) put a piece of fondant there, underneath the outer layer of fondant, to fill out the rounded part.
You might want to spray a flower nail with pam spray and put it in the bottom of the bowl, to work like a heating core. It's always hard to get the middle of the bowl done. Also, since it's glass, I would bake it at 325.
I too have used Pyrex bowls. I agree that the flower nail or a heating core will necessary to get a fully baked cake.