Haven't spent a great deal of time comparing things, but the STL Amphub is pretty damn spot on and feels like a similarish experience to the real thing. I can get similar tones with Fractal but the experience feels pretty different, which sort of leads me astray to different tones. Sounds cool though and can do the same sounds (just way more that the HW can't too).
Amplitube sounds similar but no poweramp controls, which I think the JMP really needs for my scooped leaning ears. At first I thought Nembrini was MILES off but it turns out its just because I blindly copied settings from the HW, ignoring the fact that they didn't put an output knob, so you have to use the channel volume to determine how hard you slam into the power amp. What I like best with Nembrini is the look, and the controls have the same ranges and increments as the HW. It can be a bastard to use, but it gives you results and an experience closer to the HW.
Out of these, for the JMP experience, I'd probably lean most towards Nembrini, although STL can do a job too.