I own a JS1000BTB and it's a beautiful instrument.
From what you've said, you already own 2 guitars which satisfy your harder leanings.
The JS1000BTB is perfectly capable of hard rock, but it will also do blues and anything else you might wish to try.
I play in a band that does mostly straight-ahead rock, some blues and some slightly funky rock. The JS1000 does all of them.
I run it through a Marshall 100w Valvestate stack, and through that I'm disatisfied with my tone but that's down to the amp. It's still fairly new and sterile.
At home I have an old original Valvestate series 40w combo, and the JS
sounds so damn good thru that..
I personally think that "tone" comes primarily from the fingers, and is flavoured by the wood and pickups, though not dominated by them unless they're extremely characterful.
I know for certain that the JS1000BTB is a very versatile guitar, with some great tones inherent in the guitar, especially clean tones. With the coil tap you effectively have 6 unique tones, and the high-pass filter being switchable means you have many shades of grey within those tones with different volume settings.
I know absolutely nothing about the other guitar you like. However, you've already said that this was
your dream guitar.
Like others have said, play both guitars. I can see the JS
being a better blues guitar, and it's so small, round and light, it's like hugging a sucked sweet!
excellent for unplugged strums when you're bored.
Upper fret access is no issue if you employ a little ingenuity during your playing. And you don't need 24frets for rock or blues. That's purely for fret-wankers. :P
Again, try both
The RGT3120, with its thru neck and mahogany body, might have better sustain. However I've not found that to be an issue with the JS
(and that's comparing it against a LesPaul).
Very tricky. This is a very important decision, I'd take a loooong time deciding. Try each guitar as many times as possible - if the company wants to make a sale they'll tolerate you visiting daily