If you're looking for a device to play PGO on it, I strongly recommend a device with a minimum 3GB of RAM. 4GB is ideally for Pokemon as you won't experience hiccups or lag while playing.
Connectivity issues are a different story as they are caused by the internet connection and Niantic servers.
I have been using a OP3 since the 1st month of Pokemon Go (Been using a MOTO X 1st gen beforehand). I cannot recommend this phone enough. OnePlus 3 offers high end specs and hardware at a very accessible price. The Model 3 is priced roughly around ~400 euros (about ~50-60 euros more than Samsung A3). I have never experienced lag or hiccups with the phone (in Pokemon Go or any other app) and I tend to test the limits of my phone by running multiple apps at the same time (while playing Pokemon), record screen, hotspot, and still no lag. The only issues I've experienced while playing Pokemon GO are connectivity issues in crowded areas and in-game bugs (which happen anyway, no matter the device you use.) This has 6 GB of Ram, way more than you need for PGO, so you can settle for 3 or 4 GB of ram.
Of the two models you suggested, I still recommend OP3, but if the possibilities are limited between J5 and A3, I'd go with the A3.
Hope some of this info helped you.