In terms of background, I find Dreadball to be infinitely more believable:
- A sci-fi world with an organised sport run by an organisation whose only real concern is ratings? Sounds legit to me.
- Compare this with a fantasy world where different (and mutually antagonistic!) races tour each other's lands and are not immediately cut down by the locals. Where the level of technology means that advertising and mass communications don't really exist – so where are the fans and sponsors? Doesn't work for me.
For gameplay, I've only played Dreadball. It's an excellent game, fast, easy to learn (but hard to master) and with a variety of styles & tactics. I've heard that BloodBowl is slower and larger, which may appeal more to some people. Look for online reviews; I can only tell you about one side of the story…
I suspect that Dreadbowl has the edge on costs, since a team uses fewer players (and the Dreadball Extreme variant is currently available online for the astonishingly low price of £9.99 GBP: link
Model quality varies: some of the earlier Mantic models are made of PVC and aren't so easy to build, though later ones are really good. I imagine that GW's BloodBowl figures are technically excellent, though as always their style is something you either like or loathe.
Hope this helps. I've tried to keep this balanced, but really I'm a big DreadBall fan.