The main life issue between Li-ion and NiMH is the following: (which I clipped from one of Sparticle's links.)
Li-ion batteries exhibit a much lower self-discharge rate than NiMH batteries (about 5% per month compared to 30% month). The disadvantage of Li-ion batteries, however, is that their capacity decreases over time from the date of manufacture, irrespective of usage, the extent of which depends upon storage conditions: at 100% charge about 20% per year at room temperature but only 4-6% per year at typical refrigerator temperatures; at 40% charge, however, these numbers are significantly reduced to 4 and 2%, respectively. In other words, do not fully charge up a Li-ion battery pack after a dive, but wait until the night before one's next dive, and keep the pack in a refrigerator when not in use.
Some Li-ion batteries have been reported as having a life as short as 2 years. As noted above this can be overcome by storing your packs in the fridge. Personally I find this somewhat inconvenient (plus the wife would complain), so I use NiMH packs on my self built canister light.
The only other issue regarding Li-ion is their unfortunate tendency to self immolate if you use the wrong charger (or charge rate). This shouldn't be a problem if you follow the instructions.
