I use and love both...
but as a first vaporizer should I go for the E510 from NJOY or Model J-118 from unknown manufacturer?
First off, I have no idea if the "E510" is the same as the Joye 510/Titan/Yeti/Dura. Whenever anyone here is talking about a "510", that's what they are talking about. It's a hugely popular model so it's frequently rebranded. Just make sure you're getting an actual 510.
Overall, as much as I like my J118/VX2, I'd say go with the 510 but only with a PCC. The 510 PCC negates the battery life issues. That makes the choice come down to flavor/cart life in the J118 vs throat hit/vapor in the 510, and the 510 pulls ahead there.
Either way you'll get an awesome PV, but the 510 is just slightly better overall.