During progress, you cannot afford losing people on either size. Some encounters will allow you to brush over a mistake or two, mostly at the cost of healer mana, but you cannot generalize just like that. For 25-man, it may result in not meeting the enrage timer when a single dps dies. For 10-man, it's the same. Or healer mana may become problematic. Or if a tank dies on Stone Guard, their energy bars will ensure that you're screwed on either difficulty.
It's always been like that.
Also, some encounters are more manageable in 25-man as opposed to their 10-man counterpart.
And, some encounters are more manageable in 10-man as opposed to their 25-man counterpart.
It depends on encounter-design. Encounters that require people to perform various tasks but at the same time have mechanisms that disable some people from performing them occasionally are naturally a greater hassle on 10-man than on 25-man. You have more people to fill in when the primary people temporarily cannot perform that task. On the other hand, you have encounters that require the raid to spread out, and still everyone needs to be in healing range. Way easier to handle for a 10-man.
Communication on TS/vent is also easier for a 10-man. We've had moments where various people were announcing important things... In my former 25-man raids, it was much more quiet with mostly the raid leader and the officers speaking up - unless in cases of emergency. In 25-man, I never felt like I had so much responsibility as a healer as I now do, when I raid 10-man (it sort of reminds me of when way back, I healed LK hc 10-man on an off-night, 2 healed, me being shaman... and nobody in their right mind actually wanted to bring a shaman there, at least not before the buff had stacked up to 30%. It was exhausting. So much depended on me. It was a step up from 25-man there, seriously). If you had asked me six months ago, I'd have said "Me, raiding 10-mans? Go away, 25-mans are way more epic. You'll not see me dead going 10-man with my main." Times change. People change. Old friendships break and new ones form. The raid that you're in now may in a few months no longer be the one that you want to be in, and you might leave and form a new one with people who grew on you.
Oh and, just to be an annoying person, I'll state: "A dps player cannot fathom the levels of difficulty of a raid size." I've raided enough on my dps alts to be able to say that, on most encounters, it made no difference to me if there were 9 or 24 more people in the raid. I was just performing my rotation and pushing Hymn of Hope and the likes when told to do so. When I heal, I feel more of a difference. Some encounters compliment my playstyle and class mechanics more than others, meaning that I will be performing better and more easily on one raid size than I do on the other.
Okay, Stone Guard is an example, where a dps player has to be on their toes about chains and lighting up tiles, but that one's a classic example where having more people to fill in for lighting tiles when the usual people are chained to something unfortunate is an advantage. It's silly to see how 25-mans can light up the entire room with ease. But they get an additional mechanic to deal with, and they have more people that can fail on them (which they shouldn't but... we've all raided enough to know that people don't always do as they should, and that factor is multiplied by 2.5 in the larger raid size when you look at the average, non world top 100 area. Because the people up there fail less, independent of their respective raid format).
I suppose the point of this entire rant here is: Why do we even care? So much of what we're discussing here are entirely subjective impressions! Anecdotal evidence! Differences in setup! Picking the best 10 out of a roster of 30+ people (trust me, I've been there too), and then claiming that it's all so much easier. It's not. It's not all just black and white. There are too many variables to give an objective point of view. Which is why arguments like this are totally pointless. And yet I'm participating in them all the time, if only to counteract all the one-liners that just flame around.