Love both series. Played AC1 and AC2 as well as both Fables.
Fable I was why I bought the original xbox, and I loved the game. Fable II was less awesome, but it was endless hours of fun.
AC1 quickly became repetitive. AC2 did not, but the story line was kind of "meh."
If you are shooting for time consumption, I would probably go with Fable III. There are lots of little side games in both, but all said, the Fable series usually has so many various paths that it is almost impossible to see everything.
AC is more scripted in that respect. You choose which mini-games to take, with the ultimate goal of finding enough information to assassinate a given target.
Fable gives you more paths.
All that said, I have yet to play Fable III or AC3. But if they follow suit from their predecessors, you really can't go wrong with either.