The 15s, being a 6" wide wheel, are most likely from a later truck. The main advantage I can see is that it opens up the opportunity for a slightly wider tire, and will be more stable with a modern size radial than the narrower 16" oem wheels. The disadvantage, though, is that you will be limited on overall diameter of whatever tire you decide to run. About a 28" tire is about all you're going to be able to get in a 15" that will work well on that rim.
