308 = 8" sub
115 = 15" sub
The 115 is going to dig deeper for movies, and it also sounds great musically, so I'd go with that one. I'm sure the 308 is going to have a good sound to it too, but it won't rumble as much in the low low end. Also, how much have you shopped around? I think you could get that package for a little less. Maybe not a lot less, but a little. Call all of the online authorized dealers and see who can give you the best price. If you've already done that, well, good on ya =^) Definitely go with the 115 tho.