The 16man fights are harder and the proof is the number of people that clear 8man raids faster.
For whatever reason it is logical to assume that if an 8man group can clear the instance in 1/2 nights but yet we are the only guild to pass the tank boss on 16man (3/4 currently), 16man is harder.
In my mind its a mute discussion and generally accepted that 16mans are more difficult for the following reasons:
1) Raid mechanics at this level are pass/fail therefore twice as much chance to fail in 16man.
2) The organisation is twice as hard due to twice the number of people to co-ordinate.
3) You need to do twice the amount of recruiting/gearing (which is tremendously difficult)
4) Our officers need to give double effort to craft/buy/sell to get us geared and pay for our repairs.
Not even that but the mechanics hit MUCH harder. The damage is much higher, as well as the difference in mechanics (like 2 shields per side on 16man tanks)