I had to endure my 3rd cousins Christening last weekend (Yeah, 3rd cousin. Our last common ancestor was 5 generations ago) And I knew nobody there, it was mega awkward and I left early. Considering you are under ultimatum from your soon to be in-laws, i'd say unlike myself you have something to lose by not showing, You should go.
In stock setup the GT5 version looks better. But, when it gets smashed up, the Forza version will look better. Also, the Forza version could have a body kit and GT spec race gear, whereas the GT5 would not. I think that's the main difference between both of the games, if you are content at looking at showroom fresh car models then Forza or Gran Turismo will do you just fine, but Forza has the additional depth that comes from customisation, and the online community to boot. While GT5 will undoubtedly be a brilliant game, Forza will be better.
Also, Watch out at TGS for a release date for GT5.