If you have the lead on a really nice or HUO Family Guy I would buy that. LOTR is probably a hair better in the long run but nice Family Guy's are very hard to find. They made a lot more LOTRs and I think down the road will be easier to get.
Also, if they don't make any more Family Guys, I see the value going up quite a bit as well. There just aren't many out there, especially compared to LOTR.
Both kick ass games though, no doubt. I own Family Guy, and I don't own LOTR because I own TSPP, all three would be overkill I think.
Ronkonkoma, NY
