Rule #1 - Only YOU are going to know what YOU like. YOU have to live with it.
Rule #2 - Wheels/tires and stance can and do make or break the whole look of your ride or any ride for that matter. Plan out what you want to do, be realistic, and don't settle for something you're not 100% happy with.
Rule #3 - If you post up askin' for opinions, people will happily give you plenty of those (including me). Personally I recommend being different and seeing any OEM-like wheel on a Mustang doesn't get me really excited. No offense to anyone, I had bullitts on my fox bodies for awhile until I ditched 'em for Billet Specialties wheels. Now everyone has those. I purchased Steeda Spyder wheels with the staggered widths front to rear with machined face spokes. A couple guys have them but not very common. I like them mostly for the fact that they are designed to fit my S197 perfectly and they get me the widest wheel and tire combo for the best price, and different look. Again see rule #1. I also like a couple wheels from a company called Concept One.
Here's a great link I found to look through many different wheels for your stang. You don't have to buy from them but they carry a lot of cool wheels that fit the 2005+ Mustang. http://www.elementwheels.com/index.asp