i drove a 540 m sport and 530 m sport a while back and found i liked the handleing of the 540 better then the 530. Also it's not teribly true what hte say about a big engine getting worse millage. It is true that if you have a heavy foot, that a bigger engine will use more gas, but, if you do a fair bit of hwy and are a good citizen (i.e pretty much follow the law) lol, then a 540 has the potential to get better millage, because the car doesn't have to work as hard to move the weight of the vehicle. Whereas the 530 with the 225hp I-6 will have to work more to move the car's weight plus you... as for city, 530 may be slightly better but again comes into play the weight aspect. Also if you live around, or travel to area's like up north where there is a fair ammount of hills, then a 540 would theoretically get better millage cause of the weight moving potential again. Really you have to look at what kind of driver you are and where you drive the most.
Good luck and here is some eye candy for you.