25 is already really incentivizing from my point of view - I raided both formats this tier.
10-man is easier to manage but also comes with higher personal responsibilty on encounters.
10-man however has and always had worst RNG problems on loot. There's nothing worse than disenchanting heroic loot on our first kill because no-one can use it, while some players are waiting 3 months to replace that one fucking blue piece of gear which refuses to drop.
So:
-25man gets more loot per person
-25man has much less chance to drop useless loot - there will always be someone who can use a drop - even for offspec
I'd gladly raid 25's, I like them. But my PC doesn't share this love and it's kinda hard to find 25 competent people willing to raid at late night schedule.
Making 25's dropping upgraded loot will make them mandatory again, at least at current VP upgrade price which is 3 weeks for just two 2/2 upgraded items.