Having done both, usually as a healer, my $0.02:

10 is easier to coordinate people (on time etc).
25 has better loot distribution.. (7 of the last 8 weeks we de'd druid gear in DS-10 because we had no druid and we're cashing in rogue/dk/mage/druid tokens for vendoring while half the rest of the raid is in dire need of T13 tokens)
25 is smoother to heal, but blizzard scale up damage accordingly.
10 is easier to have a problematic raid comp for some fights. Homogenization only goes so far.
25 seems a bit rougher on more people's machines these days than it did before.

I'd rather be in a 25 group, but my home realm is so full of special kids that don't get along that so few seem interesting in trying to deal with the logistics.

I wish that they'd end trying to make dual-mode raids. Design for either 10, 15, 25 whatever and stick to it for the expansion. Having dual-mode raids is a pain to balance for. 10's are a little too small IMO. How about 15? Target something like 2 tank, 3-4 heal, 9-10 dps? Not 15/25 or 10/25 or 10/15/25.. Only do 15.

This discussion isn't new. The end of 40man raiding was also predicted to be the end of wow.