They're both harder and easier.
Here, I'll break it down by boss.
- Shannox: 10m wins this one, but only barely. Heroic Shannox is pretty easy on both, but I find 25m slightly easier because you have more people for Rageface to eat. This makes the DPS race easier.
- Beth'tilac: Again, 10m is harder. Less people handling the adds. Someone misses the spiderlings on 25, you just throw another body at them. 10m not so much.
- Rhyolith: Tie. He just sucks. The end.
- Alysrazor: 25m, but only because the caster adds shoot more often and take longer to go down.
- Baelroc: 25man is harder. So. Much. Harder. The whole "25man raids are more difficult because it's like herding casts" argument defines this fight. There are so many reasons 25m makes me want to slit my wrists, while 10m took us 20 minutes and bam! the boss we had been working on for a month is down.
- Two shards instead of one. That means more people with tormented and one of the shards is in ranged. And it loves healers.
- Decimation attacks come way, way faster in 25m. While you have more healers, they're still split over 2 shards and the tanks. The quicker attack time means you can't recover as quickly if a healer gets tormented.
- 25 people running around like chickens with their head cut off. 10m has SO MUCH ROOM. 25m means someone sneezes and half the raid has tormented. Or the guys with Countdown have to run through people because there's not enough time to go on the outside of 23 people.
- Sure, if we were a perfect guild with perfect reaction times, 25m would be no harder. But we are not, so 10m seemed like normal by the time we tried it.
What's that? Mist of Pandaria talents? Lalalala I can't hear you. I refuse to make judgments until it's way, way closer to the actual expansion. Or, since I sold my soul to Blizzard for Diablo 3, I withhold judgment until I get my MoP beta invite.