Really they both have their positives and negatives. You can't see opponents online, but they can't see you either... and it's pretty easy to remove bet-timing tells
if you're disciplined enough to take the same amount of time to check/call/bet/fold for every hand you play.
In live, your opponents can see you, but you can see them as well. This is especially useful if you're very good at schooling your own features and movements while also skilled at reading your opponents, amateurs that are used to online, but try live tournaments are easy pickings for live veterans/pros.
That being said, I would say it depends on the person for what is easier or harder, you should just know that with experience and trials with either, you can become skilled enough that both will be easy.
For me, online is easier because I dont play a lot of live. The few I have played were difficult because I couldnt school my features well enough... although I think I've been getting better based on the small time home games I play.