Fallout 3 1st, but you definately have to own Fable 2 as well. I would rank them about equally but I think Fallout 3s side quests are better whilst I found the gameplay in Fable 2 to be a little more engaging.
Overall if I had to rank them it would go
1. Fallout 3 (Personal game of the year)
2. Fable 2 (2nd best game I played all year)
So I think either way you'll be happy.
Tease.