We also recently compared the ULS15's and the f15, we had dual uls15's and a single f15 so only hooked in a single uls and a single f15. The ULS sounded better hands down, no comparison... It sounded like it had more output, easier to place, better build, and sounded better with music. I say it all the time the uls15 is about the most musical sub I have ever heard, I met Dr. HSU also {didn't have lunch lol, but spoke with him in a hotel room

, and he is a very smart person, I don't know why that ULS15 is so musical, but it just sounds better than anything I ever heard, and the old thought process was smaller sub for music, he threw that out the window for sure.... Then when you use dual uls15's it gets even better... For music I am very picky and the sub either does it or doesn't, output isn't of huge importance since it doesn't take much to keep up with 6.5" mains, but you need that crystal clear, noiseless reproduction, bouncy, perfect bass... Not like with movies, that rumbly mechanical noisy reproduction will never get picked up on, while listening to a car exploding, but when listening to certain music you want it to sound like a drum is making that note, not a speaker and the uls does it better than any others I heard...
As far as the price, with the uls15 on sale its cheaper than the f15 if you need xlr's, but they are so close in price its not worth mentioning, what is $50 when spending a $1000, I just spent $148 filling up my truck with fuel that will last 3 days...
You want to compare subs and see which falls apart listen to godzilla {blue oyster cult} and this one
https://www.youtube.com/watch?v=aHSYXt1iEpE , they aren't super low bass, but they have something that all subs don't do well, I also use some eminem tracks but that is easy bass for some reason the bass in bad bad thing sounds good on some subs and bad on others....
But thats the point we all like what we like, I hear what I hear so spend my money how I want and you do the same thing, that doesn't make you wrong, just makes you prefer shoddy bass vs clean and clear, jk

the rythmik is a fine sub, just for my money I prefer the HSU... If I never heard the uls I would be happy with the f15 for sure, I just know that usl15 to be super musical and a powerhouse...