What is better 1 bigger sub or 2 small ones

I'm afraid to say that there are some tradeoffs that should be considered. Many will claim that multiple subs are always better than a single one, since it will provide more even bass throughout the room. While the bass uniformity will be better with multiple subs, the speed and depth of bass reproduction is also dependent upon the quality level of the sub(s) being used. So there are cases where a single higher quality sub will outperform multiple lower cost subs in a given room. I don't believe that there is a hard and fast rule about this that will apply to all room setups.

(1) This is entirely room environment dependent. Without actually test-driving the alternatives personally, you are only guessing .

(2) Multiple subs are harder to set up properly and

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

As with most things...it depends.

Good larger subs generally go lower with flatter frequency response. A single sub will sound more different in different parts of the room in many cases, so there is more of a sweet spot. Two subs enable more balanced response in more locations within the room concurrently.

Also the right sub depends on what speakers used with. To fill in the low end effectively with speakers that are more full range already, you will need at least one large sub.

So to get the best of everything in theory you want two larger subs. Good ones two.

If cost is a factor, then you have to determine what compromises you are OK with and decide accordingly.

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.