The pulling power is essentially identical, and a 3500 SRW is little different than a 2500 (check the spec details, though). But for a 15,000 lb 5W, you need to be concerned about the weight on the hitch, which falls almost 100% on the truck rear axle and tires. A 5W puts 20-25% of its total weight there, so odds are strong that the truck rear axle needs to carry upwards of 3000 lbs in addition to whatever other weight is onboard, including a 100-150 lb 5W hitch. It is likley you will need a 3500 dually to meet that requirement.