Hi Ive owned both the 6P and G5... Ive currently got the G5...
You need to know that the 6P is pure google, which means no frills or fancy UI like Touchwiz and it can be quite basic... youll hase to D/L most apps, while the camera is brilliant, it is purely a Automatic UI.. there is no Manual mode at all... but you can d/l a 3rd party app from the play store, like ProShot or Cam FV5... the front facing speakers on the 6P are VERY loud and great sound... but not quite as full rounded as the HTC M8, but they are Good

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Coming from an S5, the 6P has a 5.7" AMOLED screen.. which means the phone is huge... great for movies and media, but can seem a bit big at first... and if you buy the 6P unbranded and unlocked you get the latest version of Android and security updates on release day... Android N is already out for 6P owners
There is NO sdcard, so if you like alot of photos or videos or music on your device... get the 64 or even the 128GB version.. It is a great phone, but personnaly for me, it was a bit on the big side and I found pure google abit too plain.... the 6P is great for 3rd party roms and tinkerring..
Now the G5 is different in every way.... it has a dual camera setup, an SDCard slot and tbe single 2amp speaker is very loud and surprisingly good... its also 24bit, so you can use it in your media centre to play hi-res music..
The G5 is much smaller than the 6p and easily managable, it also uses LGs UX5 skin, which is quite nice.
Now or some of the reviews online... the G5 is not hideous or ugly as the Samsung kids keep saying it is.., it has 2 camera eyes on the back which can give the G5 a robot look, and as everyone and their dog has an iPhone or a Samsung, the G5 looks unique and actually quite cool... Ive personally never had any issues with the anodised back cover looking like plastic, there are alot of complaints about this... I like it.
However, one thing I will say is If you do go for a G5... before you buy check that the battery base where the module meets the body of the phone, there are no big gaps. but apart from that... Its a stunning phone...
Both phones are awesome in their own way and very fast and smooth .. the 6p is Big, Its Loud .. but its pure Android with no frills or Skins, it has no SDcard and youll probable have to download most apps to the 6P
The G5 is smaller, but looks very cool.. has 2 amazing cameras and has Hi-Res audio...
Hope that helps