Ive got a makita twin 18v, it's ok. It lacks sufficient power to cut the grass when it gets long, so ideally you need to keep on top of it. Ive probably got 80 to 100 sqm of grass and it doesnt use much of the batteries. Maybe a quarter of the 5ah. Its light and quiet, 18"ish cut
I would prefer a petrol.
I have a Makita twin 18v as well, used for quick edging when not using the ride-on. I find it brilliant. It struggles in long grass, but for 50sqm I wouldn't look beyond cordless. Get whatever suits your batteries.