I been running my iPhones (4S, 6, 6S, 7+) since 2014 against a Garmin 60CSX, Garmin 64, old Garmin eMap, Garmin Edge 500, Garmin Edge 520 (not all at once but usually a phone, a handheld AND an Edge device at the same time) and found in almost all cases the iPhone does better. The newer the better.
I'm not sure about Android, but I know iPhones seem to have a filter on them that isn't effective for activities comparable to geocaching (finding an "exact" point). It seems to stop the "jitter" when you aren't physically moving much and just shows you as stationary. On the other side, Android seems to have a much bigger lag in showing your actual position while moving.
I rode in a helicopter for 2 days recently and ran the iPhone, Garmin 64, and Samsung Galaxy Tab S2 side by side with great results from each of them (Tab gave the worst results). Granted you have a good view of the sky (but for the metal helicopter all around you). The lag on Android was comical because we'd be circling something tightly and you'd be chasing the point around on the map and not have any idea where you were. Battery life was best on the tablet, worst on the Garmin 64 and the 64 was left to sit with the screen off. Tablet and phone in constant use.
Battery life with GLONASS vs GPS only should really be negligible as whatever device you use, it's gonna last about a day. If you need more than that, you should bring spare batteries so you don't lose your data.
China has some strange scrambling they do on satellite navigation too and manufacturers have to abide by this. It's really hard to map things accurately in China with consumer satellite devices so whatever their systems are, seem unimportant to me, especially since I'm not there
Here are some photos from the helicopter stuff.