10-Ring, the difference is the P60 lamp produces 65 lumens of light for one hour, the P61 will generate roughly 120 lumens... but for 20 minutes.
At the end of the day, only you know what's "worth it" or not. It all depends on what role the light will play, and how you expect to use it. If you're working in close quarters, like changing a tire, walking the dog, or writing a traffic ticket, the 60 lumen lamp is fine. But its power in terms of 'reach' falls off very quickly, compared to the 120 lumen lamp, that will punch out a surprising distance. The P61 lamp is like a mini-searchlite, capable of illuminating the side of a building very well.
For me, I changed over to the 120 lumen lamp, because in the kind of scenarios that I forsee needing this light in, the extra punch is important, and I have other lighting sources for long-term, lower level needs.
The only concern I have regarding the P61 high-output lamp in the Nitrolon case is that if you DO run it for any length of time, it's said that the lamp can get so hot, it might melt the lens and/or case of the flashlight.
HTH.