Look at the upgrade path and hitches. If you get a 5th wheel now, you will install a 5th wheel hitch in the existing truck which is costly. Within a very short time you will be out of that truck and reinstalling the hitch in a 250/2500 or 350/3500 truck and paying again. Most trucks come with a rear receiver hitch, so if you get a newer truck now you hook up to the bumper pull and go, with maybe a brake controller installation.
Now the real ringer, people looking at your 1500 with 5th wheel hitch holes in the bed, will want to degrade its value!!!! Second cost wammy! Chris