Keep in mind too, the new S550 GT's are reported to be even faster than the Boss 302 on the track. The GT Premium with the performance pack is about the same price as the Boss 302 on the used market.
Depreciation will be more on the S550 but it may be something to think about too.
I think the GT500s will hold their value well. Tuner cars seem expensive for what you get, and they suffer from depreciation in a big way.
I'm in a Mustang club, and no one is getting new tuner cars, but they are buying GT500s. Also as mentioned, the magazine test of the Rousch was underwhelming. I honestly think the Ford engineers working on Mustangs now are superior to any aftermarket manufacturer. Just look at what they've put out over the last few years: 5.0, Boss, Laguna Seca, GT500, and now the 2015 Mustang.
http://www.svtperformance.com/forums/showthread.php?785363-2012-Shelby-GT500-vs-2012-Boss-302/page3&s=786062007cf9b6d63213369e39280ea3
Seems it is a toss up all over, more power and more weight in the GT500 package
https://search.yahoo.com/search?p=2012+boss+302+vs+2012+shelby+gt500&ei=UTF-8&fr=chrf-yff26
http://www.caranddriver.com/reviews/2012-ford-mustang-boss-302-test-review