I owned an Ultra + for years, sold it (bad idea) bought a powerball (blah), sold it bought a dual recto (buzzzz), sold it bought a 3120, really liked it, loaded it with 6L6s and flat out love it! i also now have the 5150 (which i've played many times in the past but finally own my own) which has a bias mod and choke added. so i guess i really know these amps. i've also played the JSX but it was a bit too brief and years ago to honestly comment on it. from what i remember it was like a mild version of the XXX in simplest terms.
anyway, the Ultra +, XXX and 3120 are pretty much the same with the Ultra not having separate EQ controls for crunch and lead, but it does have built in spring reverb which is cool. the XXX and 3120 are pretty much the same aside from looks and the XXX shipping with 6L6s and the 3120 shipping with El34s.
the 5150 and 3120 are quite different. the 5150 clean is hardly passable while the 3120 actually has a pretty great clean channel for a high gain amp it's even better with 6L6s over the EL34s IMO. the 3120 rhythm channel has way more gain than anyone could need yet it doesn't have the top end sizzle or fizz the 5150 has. the lead channel gets even crazier but i honestly don't remember the last time i needed to use it, that's how happy i am with the rhythm channel alone. i always say it's a bit more refined high gain when you compare it to the 5150 or dual recto for example. it's mean and aggressive for sure but just not as harsh in simplest terms. the addition of the tight, mid, loose switch on the 3120 to control the low end is killer, you can make it crazy tight if you want or nice big and fat (mine is always on loose for a huge low end).
both the 3120 and 5150 really come alive with some form of screamer in front of them with the gain down, level up and tone knob to taste. it really helps shape their tone even more. that seems to be the case with all high gain amps though, my powerball liked it, so did the ultra + and dual recto.
given the choice to only have one i'd keep my 3120 over the 5150 for sure. it just does a lot more and does it all just a bit better and smoother without the harshness so it can cover more ground than just tearing your face off

. not that the brash harsh nature of the 5150 is bad, it kills, it's just different. i'm really having fun with the 5150 and am glad i bought it since it's more than enough difference between it and the 3120 to justify both.