Which one is better? I'm only F2P player ,so which one should I buy ?
Go choose mini box instead
Land of Titans ofc
IMHO, 11th is the better one.
12th is fairly ok too, but the card set from 11th main box, remain stable even after the banlist and skill nerf.
I'm F2P too, and honestly I would recommend to avoid main box as F2P. I got complete set for both 11th & 12th like 2-3 weeks after it 1st released respectively, but that also with the help of large gems reward from KC Cup & WCS.
Otherwise, you may take over 2 months to just get complete set of needed cards from main box, which by then meta may have changed.