I think it depends a bit on your planned hunt type and locations, but from what I've gathered in this thread I think you'll be happy with the 15s. I'm also a midwest guy and primarily hunt with one friend. He got 15s so I got a spotter so we could cover everything. It works well, but wish I had the 15s for around home since most AG land is half mile or less. I also like them in rolling foothills, coulee type country. In those conditions it's nice when you spot a group of deer at 800ish yards and can tell if it's worth getting closer.
For the low standards guy I think 10s >> 15s >> spotter should be the progression. Most things I find with 10s (1-1.5 miles) I could tell sex/frame with 15s. Eventually (1.5-5miles) the 10s aren't enough and you're finding stuff with the 15s. that's when you'll need the spotter to pick sex/frame.
FWIW I just bought some 15SLCs and based on some local glassing am pumped to use them this fall.
