I'm not too fussed on the dfx in the ashdown, i'd have preferred spring reverb, but you can just not use the fx...
But yeah. The volume difference between 50 and 40 watt is negligible. And different speakers have different efficiencies as well, so you could more than likely make a 40 watt amp with an efficient speaker (say a vintage 30, it's 100 db efficiency, i think) louder than a 50.
and a 40 watt valve amp, assuming you like power tube distortion, will be louder than a 50 watt solid state amp, like a valvestate.
so yeah, I agree with sifu- it's worth it to save up another £50 or £100 to get an amp that'll last, rather than buy something at £300 that you might get sick of.
Bear in mind, those ashdowns are chinese-made, along with the valveking. I've heard a few dodgy things about the reliability...
It's up to you really. But I wouldn't put as much stock in what other people say, to be honest. You should go and try out everything you can get your hands on. Bear in mind what other people have said, but at the end of the day, it's up to you. Also, if you're trying out valve amps, remember to (a) crank them (b) play with lots of dynamics and (c) see how much difference the guitar's volume control makes. If you do, odds are you'll prefer the all-valve designs.
:drink: