It would help to know what you are towing, but I don't think there is much comparison between a gasser from 2005 and a modern one.  They will tow worlds differently.  I think if you give it an honest comparison, you would find a modern gasser (properly equipped of course) will tow as well or better than your current truck.  A modern diesel will be an entirely different beast all together.
As far as it making sense to keep what you have or have a payment, you have to evaluate if you are spending as much a month on repairs/upkeep as you would on the payment.  If so, it starts to point towards new or newer, but it is still not that simple.  A newer truck might be more expensive to insure, register (don't know if your registration is age based or weight based), etc.  You also have to think about what might be coming up as a major expense.  For instance, we had a van that the AC compressor was going out in.  Repair was going to be between 2700 and 3000 depending on which compressor we went with.  The van was only work about 5000 and had a 190K miles on it.  It just didn't make any economic sense to spend that much on the van knowing there were other major systems ripe for problems due to the mileage.
One option:  if you are happy with your current truck, figure the cost of a new one and what the payment would be and start paying yourself that now.  Then when you decide to buy something else down the road, you're already accounting for the payment and you have a nice down payment.  You could use this as your repair/replace fund for the current truck.