Should I buy the BMW 550I or M5?

I'm buying my first car next month & my mom as a co-signer & I was just going to buy a 2006 550I for $26,000 for $423/60months but found a 2006 M5 for $40,000 & the dealer is going to let us have it for 696/60months. Now the M5 is $273 more a month but I don't pay any bills right now(cause I live with mom)& in about 4-5months I'll be getting a place with my cousin & I've done the math for the car payment, my half of the rent other bills & I make enough a month to pay everything & still have money left over to do with as I wish. My mom is like $273 is a lot of money that I can use to do better things with or just save at use for when I find a get I want to marry & have kids plus the their the same car M5 just has more HP. So what do you guys think I should do, get the car I'll have fun with as a first car or be smart & get the cheaper car & save?