When I was shopping for a new 5 at the end of July I faced a similar dilemma. I was going to order a 2013 535i but the end of the model year deals on the 2012's 5's were just too enticing to pass up.
The dealer that I was working with had a 550i on the lot that I fell in love with and was offering a deal that seemed almost to good to be true. The final price was about what I was going to have to spend on a 2013 535i once I got it equipped the way I wanted. The only problem was I really didn't like the wheels that came with the sport package 550i. In fact, of all the wheels BMW has offered through the years these would have to be near the bottom of my list.
So decided on the spot that the deal was to good to pass up and I would just replace the stock wheels with aftermarket wheels where you have a basically unlimited choice of styles and sizes especially since none of the wheels offered on any the 5's really do much for me. The ones I liked best were the 2012 19" 535i sport wheels but they don't offer them in 2013.
I ended purchasing the car and ordered some aftermarket wheels within a week of picking it up. They just arrived this past Monday and I had them installed the same day. I have to say I'm beyond thrilled with the new look. 1000% improvement in the look of the car and they about 10 pounds lighter per wheel. The thing that really surprised me is how many co-workers came up and gave me unsolicited complements on how nice they look on the car.
Looking back, I have absolutely no regrets going this route and would do it again in a heartbeat.