All the raid achievements need to show which tier of difficulty you obtained it in. That is, there shouldn't be two achievements requiring people to do both 10s and 25s for completeness, but your achievement should show something like:
[Heroic: Sha of Fear]
Requires 1 of 2
Sha of Fear (10 Player)
Sha of Fear (25 Player)
That gives you the epeen factor. If you need anything else, make it so the raid size impacts which titles you obtain.
stuntz' idea above is also decent, with some tweaking. 25-player loot could start one tier of upgrades above 10-player. Both drops would need to have the same upgrade potential (i.e., they can be upgraded to the same top level), but 10-player loot could take one extra set of upgrades.