"best" and "most reliable" are completely subjective.
they're both good brands.
The argument here is really that of the whole PC vs Mac. There are companies like Anker, Jackery, and 100s of others on Amazon that use cheap components, overstate their capabilities, and go for cheap, but work perfectly fine. We'll call those "PC" ... then there's Mophie which makes a better quality product, has higher-end distribution channels, and world-class support should something happen... but all of that comes with a steep cost... we'll call them "Mac"
Honestly if you just need power on the go, cheap isn't necessarily a bad thing... worst case if it dies in 6 months you replace it and it's still cheaper in the long run. I've always preferred Mophie because they're such high quality and if ever there is an issue, one phone call and they send a replacement.
However, lately I've been leaning toward a couple others... when I was in Bangkok last week I bought a model that looks EXACTLY like the form factor of the iPhone 6 and packs about 12,000 mah of power, and cost me $12...
... so it really comes down to personal preference. If you're in warranty, go for whatever gets you the most bang for your buck. Personally I think it's funny people carry around huge power packs. I just need to get 1 charge out of mine, so prefer light and compact.. but everyone is different.