hi, i have one extra original and one extra 3rd party one..
from my experience.
3rd party:
Pros for M9 user: longer battery life for sure. Cheaper.
Cons for m9 user: battery meter doesnt work until u completely drain it and charge it full again. m8 doesnt have this issue i heard. but m9 definetely have this issue.. as i have it, search the internet theres alot of stuff on this.. this made me buy an original one.. and use the 3rd party as spare
the oddball of the battery meter not working is that.. u can still take pic when u have "low battery" but u will always get this message and never know whether it is out of juice until u completely drain + charge..
fears of 3rd party screwing up your m9...
original m9/m8 battery for m9 user:
Pros for m9 user: No battery meter issue as it is original. More solidly built, can feel it in my hands.. as such i do prefer the original batts i have haha.
Cons: exp.. and i mean real exp, can possibly buy 4-5 of the 3rd party one iirc. shorter battery life.