I think 18650 are the a favorite. They're lighter etc.
There's probably some way to work this out with a formula, but I was thinking the other day. Comparing the Ah cost between lifepo4 and 18650 li-ion.
If you charged 18650 pack to 80% or 4v per cell instead of 4.2v I think you'd get nearly the same, or more charge cycles than lifepo4?
Then all you'd have to do is build a pack 20% more cells than you need, then always charge to 80%. Surely that would still be cheaper than lifepo4
Just something I was contemplating