I bought Family Guy because I already had a Simpsons Data East in the line up and I'm a bigger fan of Family Guy on TV now. I'd like to own a TSPP one day.
As Pinballwizard79 said, whichever pops up first. For me, finding a FG was harder as it was rumoured that there were only around 20 Australian delivered machines in the country.
Try and play both and then see how you feel.
Cheers!
Adam.