Trail Seeker-
Man, it’s like I just had this discussion with myself yesterday, and last week, and pretty much all the time!! OK, so the following is merely my experience and my opinion so please take all with a grain of salt.
I am 6’2" at about 180 and have struggled with size on the majority of my bike choices. Some bike companies have a 19" that is too small, but their 21" is slightly too big. I have had most of the mainstream bikes out there pretty much in the following order:
20" Cannondale
21" Giant
21" Jamis
19.5" Trek
20.5" Giant
21" Jamis
19.5" Trek
19" Specialized
Short of getting a custom fit bike, I think I have tried everything!! I stopped at the point of Specialized because I like the geometry they have with the slightly longer top tube than most companies have. This allows me to have a shorter seat tube and more clearance. Plus I don’t really have to play with the length of the stem (standard 120mm for the frame), stack height, saddle placement, etc.
I currently have a 2004 Specialized Stumpjumper Comp and absolutely love it! Because of the more aggressive slope to the top tube it climbs extremely well and gives me a nice amount of room for downhills. It is by far my favorite bike I have ever owned. So good in fact that I am currently building up a 19" Specialized Epic Marathon. The geometry is perfect for me and I have no complaints.
With your height and riding style, I would say the 15 might be a little tight for you, but since size is a personal option you would know better than me about what fits you the best. Do you have a chance to actually ride it before committing to buy it? If so, I would ride as much as possible before buying it and being unhappy with the feel and size (I’ve been there too many times!!)
Hope this helps out a little bit for you. Good luck with your new (pending) sale!!!