Ha! Told ya the Valvetronix amps are good. :)
What Dave says is pretty much right though. I still do prefer a good tube amp to the valvetronix ones, but then I'm not comparing apples with apples either; I'm talking about $1000+ valve amps. I haven't A/B-ed a cheap valve amp with a valvetronix of about the same cost.
yeah, +1 on the expensive valve amps thingy too- i tried a marshall dsl401 the other day, and I wasn't overly impressed, I have to admit. didn't get to crank it, so I assume it'd sound better turned up, but, yeah...
that being said, if you can get yourself up to engl screamer/laney tt50 kind of cash (£600-£800) sambo, valve amps start to get awesome pretty quickly, lol.
but yeah, unfortunately at the moment sambo (and i'ts not your fault, i know how hard it is to get to try all the amps you want to in the one place) you're trying all of these things on their own- you need to get to try a valvetronix next to a screamer, for example.
that being said, if you WERE to go for the valvetronix, assuming you wouldn't be using it to gig, i'd probably go for the 15 watter, since it's around £100- that way you haven't wasted much cash on it if you do decide to go for a dearer valve amp later, and either way you have a pretty nifty little practice amp. Of course, if you want one to gig with....
back to square one, lol- modelling or valve!