I have a couple thoughts for you -
1. Regarding your comment about having to take 5 minute showers - If your old heater was leaking badly, you may be pleasantly surprised if all you do is just replace with a like model. Maybe the anode rod hasn't been replaced in a while, which could cause lots of rusting, not to mention a lot of gunk in the tank. Leaks don't help when you want all the hot water going to the shower. And the new model is bound to work more efficiently compared to your current one.
2. I take 'leisurely' showers. So when I head for the shower I flip on both the propane switch as well as the electric switch. I have not run out of warm water yet - but a lot does depend on input water temps to the tank.
3. Consider getting an Oxygenics BodySpa shower head. They have been discussed on these forums and you should be able to find several threads if you do a Search. Most people seem to really like them. The water does come out of the shower head 'differently' then with your OEM unit - more force, louder, aerated. They seem to work well with lower water pressures. Amazon has them and I see CW has them for about $30 right now. They come in brushed nickel or white. I've had mine for about 2 years now (fulltiming).