Yeah, so I modeled the two subs, and agree with what you found. Basically, they model pretty similar in a 4.7 cu ft box, but the UM is capable of roughly 3 dB more output before Xmax.
I tend to model a little differently for a sealed enclosure. I look at the lowest frequency that I expect the sub to play, say 20hz, and determine the power it takes to hit Xmax at that frequency. What I came up with for these two subs in your 4.7 cu ft sealed enclosure (with no polyfill) is about 210W 14mm Xmax @ 20hz for the HF (101.5 dB), and 430W at 19mm Xmax @ 20hz for the UM. (104 dB).
The Qts of both are pretty similar in your enclosure, (0.75-0.78 with no polyfill, or 0.64 to 0.67 with) so the response curve is nearly identical, but the UM outputs more if you have the power available to drive it. So if starting over again, the UM might be a good choice, but it’s not worth it at this point to switch subs.
Now, if you build large ported enclosures with your HF’s you can get MUCH more output. ( I’m assuming your output now is Xmax limited) Say, a 6 cu ft enclosure (unfilled) with a single 6” port (dual flared) tuned to 17 hz. (23.5” long port not including the flares) You hit 14mm Xmax at 25hz at 240W, but you have a dead flat response curve down to 20Hz due to the port contribution, with a –3dB point at 16Hz. Your output jumps to 110 at 20hz versus 101.5 with your current sealed box.
I prefer sealed subs, but if you are looking for more output, that is how you can get it.