Well in my opinion I have both devices.
Newer devices should always be considered an upgrade. In the case with the Zmax series it may be a toss up.
The Zmax pro is .3" larger in screen size and of course 1080 hp. Fingerprint sensor and a better stock camera than the original.
The Zmax original has a louder speaker and the overall sound is crispier. Could be the mixer.xml hack on the Zmax. Also the battery drain is a bit less consuming on the Zmax as well. My significant other reverted back to her original Zmax and prefers it over the Zmax pro due to performance and lag issues on the Zmax pro. Root ,roms,twrp.etc was very difficult to let go of. Wifi band 5ghz on the Zmax .Quit frankly im using my LG stylo on 6.0.1 rooted untiil or if the Zmax pro gets a development team like the Zmax had. Great community it was.
To get dev support on a non Nexus device was huge. The odds of it happening on the Zmax pro knowing the phone is exclusive to Metropcs. Unlikely.
My lg g stylo T-mobile variant but unlocked of course and the 1st Zmax both running 6.0.1 rooted out performs the Zmax pro in real time.
I've noticed since the Zmax pro stock settings set by the oem is set to interactive wich prevents changing the governers to performance and so on. Therfore lag when multi-tasking and heavy use. Those of you how are not technically inclined wont feel the difference nor understand this post.
First, what has your significant other done to actually cause lag on the Pro. Second, I have the LG Stylo as well, and in no way does it outperform the Zmax pro in real time, no way. Everything I do is faster, and more fluid. I can have 20 apps open and active in memory, and I don't get any stutter whatsoever. The stylo struggles when more than 3 apps are open. Gaming, although not on par with say an S7, or G5 is also fluid, and settings for graphics can be set much higher, and have less frame rate drops than that of the stylo. This is the first phone that I've had that I am honestly OK with no development on it. I've always wanted something, especially on the stylo, because I wanted better performance. Don't need that here on this one, but it is fun to tinker.
Pros: Fast, fluid, 1080p, crisp display, fingerprint sensor is actually really good, Camera 13mp pretty good pics. Gorgeous design, excellent battery life(better than the stylo), 32gb, 2gb of ram but runs at a higher speed than anything that is on the snapdragon 410(higher memory bandwidth), accepts 256gb SD cards, dolby sound equalizer. QUICK CHARGE 3.0 (have to buy 3.0 adapter), phone charged from 40%-full in 20 minutes.
Cons: none. Just kidding. Lowlights pics are bad, speaker volume is a tad low, outdoor visibility although not bad does not get too bright(stylo has it beat there), color reproduction is not accurate (not noticeably bad, but could be better), screen flickers occasionally while playing graphics intensive games, performance drops when charging, no nfc, and metro pcs bloat(but that's all metro phones). Included charger not quick charge capable, have to buy your own.