Why?

Because the Family Guy characters are utterly Texas-chainsaw-massacre-psychotic while The Simpsons characters are merely crazy.

Peter is prone to random acts of senseless violence, has had plenty of practice with the chicken, has at least one confirmed cold-blooded kill with Yogi the Bear and is Irish. Homer is generally happy and only fights when he has to. He’d also get tired really quickly and get distracted by the nearby doughnut store, run by Apu and Mort.

Marge vs. Lois. Sure, Marge is Police trained but is still no match for Lois and her kick-ass karate moves (she karateed super-powered-up General Zod back to the Phantom Zone at one point). She too has way more issues than Marge which will give her even more of an edge.

Meg vs. Bart. Bit of a close call really. All that endless set-yourself-on-fire-and-jump-out-of-the-nearest-plate-glass-window rejection has got to generate a lot of issues that you would think would give her an edge. Her parents also aren’t exactly thrilled with her existence which doesn’t help. Still right call on this one, she’s desperate enough to try to do Bart rather than fight him. Bart would kill her while freaking out. He is only ten something (still) and is still unsure about girls.

Lisa vs. Chris. Neither would fight. Chris is too nice and Lisa’s a pacifist. Lisa might even get to like Chris enough to end up going on a date with him instead. She may be younger than Bart, but she’s much more mature when it comes to relationships. Actually it’s weird that Chris is the only one out of all of the Family Guy lot that can actually be considered “normal” despite his family.

Maggie vs. Stewie. Tough call. Stewie may be the next megalomaniacal anti-Christ with several confirmed kills and many more who were killed indirectly by his actions but Maggie might actually be his match in practical smarts and actual combat as she may well be a lethal little mysterious sociopath. Maybe while Stewie does all the classic bad guy mistakes of yakking instead of killing her with his latest weapon, Maggie would go all creepy stealthy ninja and kill him from the shadows. Or maybe Stewie might actually respect Maggie in an it-takes-one-to-know-one kind of way and try to team up with her to kill everyone else (still failing miserably with Lois) but still gets killed due to Maggie’s loyalty to her family. Or maybe Stewie, knowing of the history of all the bad guys in the past may actually just shoot Maggie cold. Argh! I just don’t know! It really could go either way.

Brian vs. Snowball and Santa’s Little Helper. Brian’s generally easy going but does get it on when push comes to shove. His experiences with the rest of his family and particularly Stewie would have done the trick on his noggin. Plus, having opposable thumbs, he’d just shoot them after getting exasperated with trying to reason with them before realising that they’re just too dumb to live.

Grandpa vs. Francis Griffin. Francis may be Irish but is not combat trained. Grandpa would get a flashback from World War II: The Sequel and gun him down before he even realises what’s going on.

Anyway, I could go on, but anyone who’s into pop psychology can clearly see where this is going.