Oooh, I don't like using fruit extracts -- more often than not, you get a chemical/perfume flavor and aroma that's not pleasant. I'd rather go the puree route -- it may take time, but it's worth it in the end.
I haven't make a fruit stout, but I did make a few raspberry ales, a cherry ale, and a craisin ale. All of them added frozen or dried fruit in secondary. The fruit in these is subtle, with a nice tart flavor. They are drinkable in two months (primary, add fruit and second primary, secondary, bottle conditioning), but much better after three or four. One of the keys when using real fruit (or puree) is to add enough -- it's always more subtle than you imagine. Last time I used 4 pounds of craisins -- there was a huge mass in my brewpot while I was heating them to pasteurize, and I almost cut the amount in half, but in the end it was a nice subtle flavor and aroma -- perfect.
Whatever you try, please let us know how it turns out.