I own a 14650 e-power, and I love it.. although I do sometimes wish I opted for the 18650 version.
I do love the 14650 one though.. fits perfectly in a lanyard. The e-power + 1.7ohm cartos are my "away from home" setup, don't like taking my VV boxmod too many places. It's a great
device, true 3.7v (off the charger gives you 4.2v), can take a 14650 battery (CHEAP) or a 14500 if you have a spacer (For IMRs, if you're wanting to use dual
coils) they also make an IMR 14650 but it's not AW brand.
I feel the e-power will probably save you money in the long run... with a Riva battery, if ANY part of the battery goes dead, you have to buy another replacement battery. If the battery in the e-power goes bad, you simply replace the 14650 battery (they are $5, $2.50 from china).. if the switch goes bad, you simply replace the switch (around $12)... etc etc etc. Plus it gives you the ability to opt for a high-drain battery if you ever find yourself using dual
coils or very low resistance.