Several fights are harder by far on 10m and harder by far on 25m. This goes for both normal and heroic (but the bosses will change).
Both 10m and 25m raid guilds have if you average it out, equal challenge.
For example pre nerf halfus, valonia were considered extreme by 25m hc guilds that had done them on hc 25. Those same guilds weren't able to do it on 10m.
Cho'gall normal 10m was way, way harder than 25m normal. Likewise, there are much harder 25m's to get through such as al'akir to name but one boss.
I have respect equally, for both in cataclysm :-)