I haven't heard the JA's but owned 30.2 XDs for 2 years. They are not thin but they do not rock-out. I don't listen to a lot of classical music but I don't think the small Harbeth's excel when there is a lot going on. Jazz, blues, vocals, acoustic, well recorded stuff like Steely Dan sound great and not fatiguing.
I bought them because I heard them and could not get the sound of the midrange / vocal reproduction out of my head. I'd listen to both if I were you you'll probably be drawn to one or the other and it will be an easy call unless you hate them both!
btw I also bought 2 REL T/9x subs at the same time as the 30's - I have a large non-dedicated room - I think you'll need a pair of subs to be really happy, maybe with either choice.
Having said all that, I now have the 40.3's and I sold the RELs. Much happier. As someone else pointed out, in your room why not go full size?
Lastly, FWIW Harbeth shows with Hegel and while the 30's worked well with Tubes for me, they really are best with beefy SS.