1st off there's not a wrong answer here. The Rythmik and PSA are a step up in performance from the HSU, however, they cost more, the HSU is high value, and is a stout sub.
The Rythmik has the output advantage at port tune and higher for a bit then the PSA has the output advantage. I forget the exact number where the PSA takes over, perhaps around 50Hz. Neither sub has a huge advantage in either region. A ported sub is not too happy below port tune, so the PSA gets the advantage way down there, although I am still impressed with how well the Rythmik handles the low stuff like Bass I Love You, although that songs deep note gets the Rythmik woofer really moving, it is still in control and nothing bad happens, though you shouldn't be cranking that song at reference. The sealed PSA will handle those notes below 10Hz more gracefully but of course you can put a high pass on the Rythmik if you want.
I went with Dual Rythmik FV15HPs and absolutely love them. They are built like a tank, Brian provides great customer service, the sound is soooo accurate, and they are very reasonably priced for their capabilities. The tremendous output capabilities are a blast, but I find myself doing a lot of -10 to -20 music listening and the sound quality is amazing. There have been some tests which showed the Servo reducing distortion; Rythmik Brian commonly talks about the greater cone control it provides.
I also own a PSA XS-15. It is well built, sounds great, great value, and PSA also has actively involved great customer service. There are several members around with dual XS-30s who will show their very high praise as well; I completely believe them. I have a lot of respect for the PSA owners, their subs, and their users.
We are fortunate to have these great options available. In the end you can weigh the minor differences and see what you value more. Or just flip a coin. Either way you will end up with a fabulous sub.
Are you not considering duals? I bought one FV15HP knowing I would get to duals within a year. If it was going to take longer then that I would have bought two lesser subs.
Hope this helps