On a 2M pattern plane usually 10 cells 5000 mAh can be a good compromise so You can fly on more throttle in windy conditions without worry about getting low on capacity (and ruin Your LiPo by over disharge them). I use ThunderPower G8 25C Pro lite+, 4 cell x 2, 5000 mAh, in my MythoS 125E and same type of ThunderPower LiPo but 5 cell x 2 on my MythoS Pro 2M. On windy days when flying Advanced Schedule I can consume around 3300-3500 mAh when flying my MythoS Pro 2M. I'm seldom below 30% of capacity so it feels good to not need to worry about not even come close to 20% capacity rule of thumb.
Above is with Hacker Q80-14XS and Falcon Electric carbon V2 prop 20.5 x 14.5.

About the form factor on the ThunderPower LiPo - 5000 vs 5400 mAh: Yes they are about same weight and I have tried the 5400 mAh (4 cell x 2 in my MythoS 125E) and it works fine but I do not really like the tall pack. I prefer the lower profile 5000 mAh. See photos below of comparison of the two packs.

/Bo