Good players (very often forum posters) prefer 8 vs 8 because it doesnt force them to mix with low skill people, and they have individually a higher impact on the match result, compared to 10vs10 when it depends more about fate.
So yeah 10vs10 was more fun, but it was also too hard to organize for team callers without whom nothing in that game could be done.
Now with 8 vs 8 new teams can pop up more easily, it is easier to gather people, so it brings fresh air to the game. I am sure the ladder system will also contribute to that freshness, compared to an unflexible league of 8 pro-team with always the same faces.
Though if we only take into account the fun once the match has started 10vs10 is better : more butchery, more killing in the back that insolent pro-player, more getting banged by 5 nord infantry right from the beginning etc etc. The more you diminish the size of the team the more the game turns into Warband Esport aka abusing game flaws with pike stun bumpstab and other bull****.
It is all a question of gameyism vs romanticism.