"I find myself at the end of this paragraph and noting that you now seem to be a new player, who didn't raid before Cataclysm, never raided progression 25 mans yet seem to have a very strong opinion about which you believe is the harder format. Bias much?"

From here http://us.battle.net/wow/en/character/alexstrasza/Malend/simple

to here http://us.battle.net/wow/en/character/Kargath/Gracher/advanced

I rerolled realms in the middle of Icecrown, I just did the 1st wing of ICC(Few guild mates switched mains over and I had less time to play at the time so I leveled a few alts). Aside from missing out on ICC, I've raided from the end of BC up until today. Both 10 and 25 man. The changes to the lock out system compelled us to go 10 man.

My current position is 6 of 8 in DS, due to the xmas break and all that, little slow getting back in the grove of things now. The guild I am in is sort of a sister guild to a 25 man raiding guild, they're working on heroic fights now and have lost 6 players during a heroic Morchok kill.

Not every boss is the same of course, just as not everyone's raiding experience is the same. Overall on 10 man normal raids, if you lose a player or 2 your odds of wiping go up by a lot compared to 25 man normal raiding.

It should be pointed out that Blizzard has said the same thing when asked about a 10 man version of LFR, 25 man raiding is more forgiving of players compared to 10 man.