I'm really no good at this advice stuff...especially in a situation like this...but I do love the Infinity G35
If you like the mercedes better...maybe it would be ok to trade it in for that and get it all paid off sooner than you had planned to with the infinity?? even if you will be upside down....but maybe since you are going to be able to pay it off faster, that wouldn't be too awful. You just wouldn't be able to get rid of the mercedes because no one would want to pay off your "upside down" loan. know what I mean?
again..I'm no good at this LOL maybe someone will have some advice for you!!