Depends on how bright you want it. Upgrading using the conversion kit puts the light output to 200 lumens.. while the P60L only puts out 80 lumens.. If you want a "cheap" method of getting of getting a high output light, then conversion kit is very cool.
I've dealt with TNVC a few times and I'm pretty sure Victor would stand behind the conversion kit or your money back. He's always dealt fairly with me.. even generously (ie had a problem he paid shipping for return and got a new one with Surefire batteries). Since I have the kit I know it works very good. I haven't tested it for durability though since I don't put my rifle through the ringer. I may have dropped it one time though.. but it still works.

Oh I forgot to mention that if you get the tail cap, it puts out different brightness on a click of a button which the P60L doesn't do.