There's a big difference here between the two units.
The SP2 is fully mechanical - no wires, no electronics. When used with protected batteries (a pair of which, with charger, can be had for less than $25) I think it would be hard to beat for solidity. It has an 801 connector but a decent adapter ($3) lets one use any 510 attys or cartos.
The E-Power looks good and the modular approach seems worthwhile - but it is still twice the price of an SP2 and one is stuck at 3.7v