biggest difference is where h1z1 only has melee, pistols, 1x shotgun, 1x ar15, 1x ak47 and 1x hunting rifle, battlegrounds has a lot more military weapons and attachments.
assault rifles (5): AKM, Kar98k, M16A4, M416, SCAR-L
smgs (3): Micro uzi, Tommy gun, UMP9
sniper (4): AWM, KAR98K, M24, SKS
lmgs (1): M249
shotguns (4): 12 Gauge, S1897, S686, S12K
while you're looting you also come across suppressors, compensators, vertical grips, stocks, magazine mods (extended + quickdraw) and scopes (red dot, 4x, 8x, 15x). some are generic, some only work with a particular gun. this introduces a lot of imbalance which worries me a bit. these things make a huge difference and the advantage will go to whoevers lucky enough to find certain weapon mods (particularly x8 scopes and silencers). in h1z1 it's a more even playing field gear-wise, it's then down to comms, positioning and getting the jump on people.
the map is 64km2 (same as h1z1) but instead of americana it's set on an island with numerous villages and towns. currently maxes out at 100 ppl per round which is less than h1z1 (~160) and with the loot areas spread out more your ttk after the landing shootout seems quite high. i've seen a sedan and a 2 man buggy so far and you kinda need them because when the gas circle closes in it's pretty fkn fast. airdrops are in the game but instead of the h1z1 carpet bombing there's an occasional red circle on your map that seems to appear randomly. if you get caught in one you have to leave or find cover before it rains mortars. it's an interesting mechanic. maybe a bit too random? time will tell.
from ~10pm tonight
h1z1: 83,684 playing
battlegrounds: 19,682 playing
it didn't kill h1z1 overnight and while it'll poach a lot of players i don't think it will. the arcadey feel and graphics in h1z1 means you aren't thrashing your system to try and distinguish grey blobs in a wash of detail. there are things i like and hate about both games. right now i think h1z1 is more fun to play. they both need work, battlegrounds just needs a LOT more.