okay unlike many here I have actually played both of them (curtousy of blockbuster gamepass, frigging awesome btw) and i think that it is a fairly close race between them, but i have been a lionhead fan since they made the origional black and white, but i friggin love oblivion, id have to say a tie.
Fable 2: fable 2 is a completely awesome game, it has everything i wanted out of it, except its replay value... the replay value is what brought this game down by the buckeloads, after you beat the main storyline you are given a buttload of sidequests, witch isnt bad at first, but after awhile you dont feel like youre playing fable 2 annymore, you feel like youre playing capatain repetitives' monotney adventure. But this game is simply amazing with the main questline, but thats not to say that this game is totally bad, once you beat it with one alignment or skill mastery, you can go and beat it again with a different alignment making the game totaly different --RECOMEND RENTING-- you can beat it in a week if you get it from blockbuster, and if you like it just ask them if you can buy the coppy from them.
Fallout 3: fallout 3 on the other hand has a very long storyline and a great replay value but did not quite give me the experience that fable 2 gave me when i played it. This might just be me, and probably is, but when i was playing I found it rather difficult, the controlls were aquward and the ammo was quite scarse, and i was constantly spending all my money on health items. But this game is quite good comming from the people who have played it and i wouldnt doubt that it is a good game (havent beat it all the way through) though it is a solit rpg game with many little surprises. --RECCOMENDED BUY-- if youre going to get this youd have to buy it because the questline is quite long by my standards (at least 7+ solid hours of gameplay but thats just me *again*)
Favor: FABLE 2
rent it and see witch one you want, or did what i did, ask for 6 months of blockbuster gamepass and dont get either, i liked it a little more than a game cuz i got like 4k gamerscore