My two cents worth... We've been to a number of RV shows in the past couple of years. We have not been impressed with the build quality of most gassers until you get into the higher end models. Some seemed just downright flimsy inside including low end furniture.
Gas is cheaper than diesel. Oil changes and periodic maintenance are less expensive with a gasser. Tires are more expensive for a DP. Most gassers seem underpowered especially in the mountains. I pass them continually. As you stated your towing capacity is limited with a gasser as is the gross carrying capacity. DP chassis and running gear are much more robust than gassers.
One deciding factor that you didn't mention was the number of days per year you would use the RV and how long you plan to keep it. If you are planning on using it say 120+ nights per year and keeping it for 5+ years, you probably should look at a DP.
There are a lot of really nice and well maintained DPs out there. If you can find one with all the maintenance records and pay for an inspection, you really shouldn't have a lot of upfront expenses.