This question gets asked often and you will usually get one of three responses.
1. Go with dual smaller subs. The advantage to multiple subs, especially in a rectangular sealed room is that you are able to get a smoother frequency response. While this is technically accurate, most people would not notice the nulls and peaks with actual content (even though a lot of people claim they can). Most content in movies is a complex combination of different frequencies and not a sustained single frequency. What might look bad in a graph, likely doesn't sound bad to most listeners.
2. Get the single biggest sub you can afford now. The advantage here is obvious in that you are buying the most capable subwoofer you can afford. Unlike option 1 which has a more difficult and costly upgrade path should you get the itch in the future, this option allows you to simply add another of the larger sub. While this option might result in some nulls throughout the room, as mentioned earlier, nulls that are obvious in a measurement graph really aren't noticeable in 99.9 percent of content. What you will notice is the additional output of a larger sub.
3. It depends on the room, listening preference, etc etc. This is the best answer but doesn't really help you make a decision.
My personal subwoofer journey looks like this:
Single 12 -> Single 15 -> Dual 15 -> Dual 18 -> Single Sub with Dual 18 inch drivers
In each of the 5 configurations, I used REW to do measurement graphs and optimize placement. The dual sub options always had the smoother looking graphs, but I am happiest with my current setup that uses a single sub. Although all of the single sub setups I have had suffered from "some" narrow nulls, I never felt like I was missing out on anything which watching a movie as a result. The things that are very obvious are amplitude and extension.