Both. Mostly speed.
Each antennae are each like a separate hardware that each could be tuned differently. one antennae is for 2.4ghz connection and the other is either or 2.4ghz or 5ghz. The range would be almost the same or more depending on how you position the antennae and which type of mentioned connections you use. I believe 5ghz is shorter but faster with more bandwidth. you control them in the “interfaces” and “wireless” part of the LuCi interface. Dont get the single antennae version. More mbps is better. 2 antennaes are better than one. Especially in cases when you are further away. You never know when you need more. Just a few more bucks and the gl arm300 is enough.
More importantly, it’s way better when you focus one antennae on connecting to your devices and leave the other connected only to the wifi you are extending/repeating. Or else you might get your wifi disconnected. So yeah. 2 antennas are better.