What is better 1 bigger sub or 2 small ones

multiple subs are usually introduced in a large room for HT ..... Not for 2 channel

Ridiculous statement.

Agree with the, "it depends", viewpoints. However, do not forget that subwoofers are speakers too. So do not percieve or treat them with any less consideration than your main speakers.

Multiple subs are harder to set up properly

I also disagree with this statement. I agree that it is possibly more difficult. But not necessarily. Once again, it depends. For example, if the room is such that placement of a single sub disallows it to ever be properly set up, that is, no matter where you place it, you have noticeable issues (i.e., frequency suck-outs and/or boominess), then a second or multiple subs might be used to eliminate those issues which a single sub cannot. In this case it is then easier with multiple subs to "properly" set up.