I have Osprey Atmos AG 50 and I am trying to buy a rain cover for it. There are two sizes in Ultralight rain cover model.One is 'M' size (upto 50 L) and another is 'L' size (50-75 L). Have anyone bought 'L' size cover? I am bit concerned with 'M' size because when the bag is fully loaded will it cover well? Is it better to go for 'L'?
Either should be fine, the M size may not fully cover it if you have the backpack stuffed to the max.
On the other hand, as of right now it's a difference of 0.2 oz + $3 between the two, so if you are worried that much about it, then just get the bigger one. The other advantage of the bigger cover is if you ever need a bigger pack for longer trips you already have one.
The bottom line is that the difference between the two is pretty negligible so I would just get the bigger one.