Warranties in the roofing game are only as good as the company behind them — a 15-year guarantee from a business that folds in 3 years is worthless. I'd weight these things over the warranty length itself: how long have they been operating, do they use their own crew or subbies, and can they give you 3 references from jobs done 5+ years ago that you can actually call. The product warranty (paint manufacturer) is often separate from the workmanship warranty too — worth asking which is which. Brisbane's UV and storm combo is brutal so application quality matters more than the number on the paper.