The Average diameter for 15lb line is around the 0.35, I've fished France for the past 4 years using either Fox camo (which I'd never use again) which is around 0.38 and then for the pass few year Adrealine which I think is about 0.33. I've never been snapped off.
What you need to take into account is the lake, what is the lake bed like, gravel etc, snags, weed beds etc, when you've got that info then you can make a more informed decision. Plus are you needing to cast a fair distance, if so you should be looking for a lower diameter line to help with this
The diameter is more important than the BS, as line is over rated, there is a Tackle box review on here somewhere which will give you the true BS of the majority of line.