link to the one you're looking at please?
18650 batteries are preferable because they've got large capacities, are rechargeable and are quite affordable.
because they produce a voltage close to that of the LED's required voltage, it significantly simplifies the circuitry too.
you can spend between nothing and £30 on a single 18650, giving you a variety of capacities and reliability, but as far as i'm concerned the AW/Redilast 2900mAh 18650's with the protection circuit are the best. the raw Panasonic cells they're made from are cheaper but less suitable for novices or people who aren't obsessive like me
Cree currently make the best LED's as far as efficiency goes, and their Venerable XR-E (model) Q5 (Flux-bin) leds are still very popular with the cheap manufacturers who have very large stocks of them.
if you want a very simple and reliable torch that uses a 18650 rechargeable, the
Shiningbeam S-Mini is highly regarded and also nice and cheap
Re. your "how long would they last" question, 18650's can be recharged from between 250 to 2000 times depending on how they're treated, but 500 cycles is quite achievable.
with an average 2200mAh cell, the shiningbeam S-mini would last about 2 hours on max, 5 hours on medium and about 90 hours on low.