I tend to find endless amusement in the people who quote that 25 mans are harder to run as justification for anything. That affects what, 3 people? Maybe as many as 6 in a really poorly run 25 man? For the average raider who just shows up, executes his/her rotation and doesn't stand in the bad stuff you can't really get up in arms about the amount of administrative overhead in a 25 man guild.
Also if you're going to argue difficulties of relative raid difficulties, you have to use numbers and normalise to something. For this tier let's use heroic madness and normal madness heroic madness for difficulty and normal to normalise to the number of guilds attempting the content.
25 man: 347 heroic/ 3001 normal ~ 10% of the guilds attempting the content cleared it on heroic.
10 man: 933 heroic/ 33000 normal ~ 3% of the guilds attempting the content cleared it on heroic.
you can choose many other metrics and normalisations but don't just say "go look at a video" as justification for your opinion. Anyone who looked at those stats above would be justified in saying that the data supports a theory that 10 mans are harder than 25 mans. It doesn't prove it, it doesn't make it fact, it supports, that's what data does in situations like this. 10 vs 25 man raids being harder or easier is purely a function of blizzards tuning of that specific encounter that's it.
As far as needing your achievement to say what raid size you killed it on? Are you frequently running into situations where you tell someone you killed a boss on 25 man and people aren't believing you? The argument should be that you are doing 25 mans because you enjoy them more than 10 mans for what ever reason and luckily that's not something that blizzard can add or take with you. The only possible place this issue has some merit is in realm 1st guild achievements, and I would be for having a separate guild achievement for each raid size. Though honestly I think it's more of a concern to add a way to tell if a guild has killed a boss w/ or w/o an optional raid wide buff but that's not what this topic is about.
Edit: I just read the reply above mine and literally giggled. "Nice try."