There are 5ah and 7.5ah versions of the Ego, with the 5ah being at the same price point.
Kobalt is 80v *2ah = 160Wh/battery * 2 batteries = 320Wh
Ego is 56v * 5ah = 280Wh/battery * 1 battery = 280Wh
Ego is 56v * 7.5ah = 420Wh/battery * 1 battery = 420Wh
You'll get more mow time out of one battery with the Ego for sure. You'll get more mow time total with the 7.5ah Ego. I don't mind the breaks, myself, it reminds me to take a break and not get heat stroke. It does slow down the process as a whole, however.
If you mow regularly, I think the Ego is likely the better choice. I went with the Kobalt as the higher voltage allows it to power through tougher parts, and I tend to be neglectful of my yard. I've noticed that 2 batteries can do my front yard when I actually stay on top of it regularly, but I try not to mow more than 5x a year!
Also the Kobalt has a low hassle 5 year warranty vs the Ego 3 year warranty, since Kobalt is Lowe's brand.
Pulled the trigger on going EV on 10/2016 with a 2012 Leaf, and a Tesla Model 3 reservation expected to receive in June 2018.