Well I own both and Fable II was a very good game, but Fallout 3 got all my attention after I've beaten Fable's mainquest. (I admit I'm a Bethesda fanboy)

If you love Oblivion, go buy Fallout 3.

Simple as that.

Fable II.

Fallout 3, Fable 2 didn't live up to the hype at all for me. It was still good but the ending really made me mad.

I say, Fable. Lots of stuff to do beyond the story.

 Achievementhunters I hate them =p.

Anyway I would go with Fallout 3.  Fable II will probably end up in a half year in the classic collection get it then, and when you have finished Fable II the Fallout 3 DLC will be there :).

 Poor argument. If anything that's Fallout 3's strongest point seeing as how  you can beat the main story in six hours or so and yet can find yourself playing the game for 100 hours or more doing all the side quests. But sure, I guess you can work at the blacksmith hitting the anvil for however many hours you want in Fable 2 if that floats your boat. Just saying fable 2 was a better game would have been a stronger argument.

Flirting with women, owning property, getting a family > side quests

The stuff that Fable lets you do is more interesing to me.

 But come on, even if you like those things you can't possibly pass them off as being deep. I mean, at first it sounds deep and in fact is why I bought the game. But it doesn't go any deeper than farting or using other emotions that again aren't very deep or fun and then someone wants to marry you. Owning property is pretty deep but requires you go up to a building and purchase a deed but it does fluctuate according to the economy which is neat. But I think to a majority of gamers doing actual missions and changing things up rather then playing around with the same emotions and buying a deed is more fun . Whoever spends 100 hours doing those things should also pick up knitting, because that's about as much fun as doing those things for that amount of time. Not to hate on your choice though. We all  have personal preferences.

Fable 2. If you like Action RPGs, this will draw you in.

I have Fallout 3 also. Tried it for a bit, will get back to it when i finish other games. I don't really have the urgency to play it ATM. Valkyria Chronicles, COD 5 and Gears 2 will probably go before Fallout 3.

On the other hand I didn't want to stop playing Fable 2 and I'll come back to it again after NXE.

Both are good but Fallout 3 is a bit better.