It's nice to have options!
In some respects, starting with a single good sub is a little easier than starting with two subwoofers, because you will have a lot to learn about how to position your subwoofer properly, and how to operate it properly. Subwoofers are a little more complicated than speakers, and subwoofer placement and settings matter.
I will leave it up to you as to whether you prefer a Cap 118 or a PB4000. The Cap 118 will have more overall output, especially in the mid-bass. The PB4000 has the advantage of already being available in the piano black finish, and you can try one for 45-days and return it for a full refund, with free shipping, if it doesn't satisfy you. For someone who is just trying to figure out what he wants, that can be an advantage.
I'm not sure what you mean by a sound dispersion problem. A single good subwoofer should do a pretty good job by itself in a 16' by 16' room. Two subwoofers should do an even better job, but they may also require a little more effort to integrate properly.
To me, the question of whether to start with one or two is literally an instant gratification, versus long-term vision/investment, question. Most experienced home theater owners will tell you that it is better to build your HT components strategically, selecting each component carefully, rather than trying to hit the ground running with something less than you will want long-term.
If you buy two HSU subwoofers now, and get curious about whether you compromised a little bit, you will have to advertise and sell them in order to upgrade. If you buy the single more expensive sub you want now, your upgrade path is much simpler--you just add a second one when you can. A few months or a year doesn't sound like such a long time to me, but I think that I have finally learned some patience about this sort of thing.
You will have to make the final decision, but my advice is nearly always to start with the best single sub you can afford and then add the second one when you can. If we were talking about having to wait years instead of months to add a second sub, my advice would probably be different.