Either the website is wrong or your looking at the wrong numbers. The 2500 will clearly tow more from a mechanical standpoint than the 1500.
I don't know what you're going to tow but if this truck is being purchased specifically for towing I would get the 2500.
I probably will never buy anything less than a 3500 truck. They are the same size, same shape, same controls, etc. but so much more capable. And, the price isn't THAT different.
I do wish all the manufacturers would go to the new standardized tow ratings but they can't agree when. If some do and others don't the ones who do are now at a marketing disadvantage so they're kind of daring each other to go first.