Most "36 Watt" amps (including mine) are based on a double 18 watt output stage, hence the name. They use either 4 x EL84 or 4 x 6V6. Each of these has a slightly different take on the sound.
They're ususually cathode biased and have no negative feedback (as against fixed bias with NFB on the JTM45). In short, this gives them a crisper, rawer distortion tone, and a chimier clean. Imagine a Vox crossed with a Marshall, and that gets you most of the way there, although it isn't as clear as that.
The 36 has a slightly different sound to the 18 Watter, being a bit fuller, not just louder.
Compared with that, the JTM45 is warmer, more rounded, and smoother.
Both sound great, and which you prefer is entirely subjective.
I've nearly finished my Roadstar 36 demonstrator, and could have it ready for the 8th if you're inteterested. Matt's also lending me his Roadstar 18. :D