I used to be a Ravpower fan for power banks but they ruined them. Their OLDER models would work as a USB UPS, which are real nice. Then they started making smaller ones that glitched horribly if you tried to do that (of course no warning in their marketing materials about that!) and now like most makers it's unreliable at best.
As for extended batteries buy from someone who has a good return policy, run a couple of full cycles immediately after you get it, and use Accubattery to check capacity. A whole lot of the batteries sold by third parties, no matter whether extended capacity or not, are crap and have nowhere near their advertised capacity. There is utterly no point in having a big bulging battery case on the back of the phone if you only have 3,200mah of actual capacity in the cell -- no different than the stock battery!