Hi is the purpose of the dual antennas on the GL-AR300M-ext to achieve range or speed? Or something else?
I notice the GL-AR300M is 300mbps but set to 150mbps by firmware default. Would it have the same range as say the GL-AR150-ext-2 which has a single antenna?
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.
I’m pretty sure neither support 5ghz…right?
I read somewhere that the power output of the GL-AR150 is 5% less power
The price different is almost double and I’m buying many which is why I’m trying to learn. Thanks 
the AR-300M is 2.4GHz only (there was a 5ghz module but not produced any longer).
TL;DR - More antennas - better signal. You can also orientate them differently to better cover vertically (up stairs - or when you rotate your phone/tablet) and horizontally.