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.
Flawed logic, by the way. There are very few 16 man guilds in the first place. On the Empire side TOFN there is NONE, and it is next to impossible to find a stable 16 man group going for a few weeks without people leaving the game in droves. The average life span of a SWTOR is appx. few weeks, and I'm pretty sure the other servers have similar issues. I'm not arguing that 16 man is easier, I'm sure there are fights in EC NiM which are significantly harder on 16 by default, it's just that your first statement is flawed, to say the least. Seriously, I've been raiding in this game since in January and I've easily seen appx. 50+ good players leave, finding a stable 16 man guild ... well, I can only respect you guys from Friendly Fire, I would say the hardest thing about 16 man op is GETTING 16 people in a raid in the first place.
In my experience, last boss TFB HM is significantly harder on 16, while other bosses are more or less the same, I would even argue that the first one is easier because there is no increase in the amound of debuffs people get from the boss (unlike Zorn & Toth), and the dps requirement for the 3rd boss is significantly lower than on 8. Haven't tried 16 man EC NiM ... yeah, on TOFN that's likely to never happen.