Quickbeam did runtime graphs on the Cree drop-ins here:
And Zven did an informal (but very well done) G2L runtime graph here:
This will give you an idea of how all three options compare in terms of battery life on 2x SF123A-type batteries.
Price, quality, output, and runtime should all be considerations. There are some reports that the Cree drop-ins won't work with 2x RCR123A rechargeable batteries, but the G2L will work on the same rechargeables. All should work with 1x 17670 rechargeable batteries, but you will sacrifice output (and in some cases you won't have regulated output with one large rechargeable). All the drop-ins mentioned claim to work up to (at least) 9v, so you could even put them in a 9P or G3 later on if you wish for extended runtime.
As for the ease of replacing the module, it's very easy. You should be aware that some of the DX drop-ins may require some fitting to get them to work in your G2, based on others' experience. The G2L, obviously, won't have this problem. As long as the module fits properly, though, changing the lamp assembly/drop-in is very easy. Unscrew the front (bezel) from the light, remove the old module, put the new one in, replace the bezel, and you're done. Maybe 30 seconds at most.
As you'll see from the reviews, there's benefits and drawbacks to all the options. The links above should give you some ideas about the different options, and hopefully that will help you out.