It depends. Yes, I know that sounds like a weaselly answer, but it really does depend. The reason to get two is to help deal with irregularities in the frequency response caused by room interaction. The thing is, what those will be, and how bad they are, will depend upon the specific room and where you put the subwoofer, as well as where you will listen to it. A reason to go with one that costs as much as the two lesser ones is that you are more likely to get deeper bass.
Now the non-complicated answer. For most people, I think it is better to go with one good one. The reason is very simple. Most people do not save up enough money to get what they ultimately want right away, and consequently upgrade their gear over time. If you buy one good subwoofer, the upgrade is simple and financially efficient: Simply buy a second subwoofer identical to the first one. However, if you bought two lesser subwoofers, the upgrade is going to involve buying two better subwoofers, and getting rid of the lesser subwoofers, which is going to end up costing you more money.
For this reason, I only recommend buying two subwoofers for people who either have enough money to buy what they really want right away, or who will never have more money to upgrade later.