Which is more profitable – an MMO with a monthly subscription or a free-to-play game?

It certainly does change things! Six months of playing WoW costs 1800 rubles if paid all at once, and 2100 rubles if paid monthly, which already gives us about 70 dollars. And that's for half a year, not a year. Don't forget about the expansions that come out every year and a half! That's another 700 rubles per expansion, at the very least.
And yes, a lot of schoolchildren play free-to-play games, and even if they want to, they won't be able to donate.