Well there's plenty of data available regarding the performance of valve amps, though you might need to dig around a bit.
Some of the high end kit that would be worth trying to get performance data on would be units from groups like Kondo, Wavac and Audionote, they tend to focus on lower powered amps. More ballsy units worth investigating would be from McIntosh and BAT (Balanced Audio Technology). Closer to home, checkout the stuff from EAR and WAD.
With regard to how bad these are, well do remember that valve amps were pretty much dead in the water a few years ago, with the exception of the real high end stuff that I've suggested some links to. It's only more recently that there's been a big resurgence with the new and cheap Chinese kit (eg. Consonance).
Against that, transistor amps exist at pretty much every price point, meaning that there's masses of cheapo dross transistor amps, aswell as the better stuff. So if you had to apply statistics, I guess that the mean price for transistor amplifiers sold would be MUCH lower.
Simply asking the question of "which is better", is therefore a bit irrelevant.
What might be a more interesting question would be "which is better at some specific price points", e.g. £100, £200, £400, £800 etc.
You could then also take into account speakers that they might well be paired at for those price points.
Lastly, there's a variety of different tube types and implementations out there. E.g. 2A3s, EL34s, KT88s, 300Bs, 805s etc. From what I've read, all of them have their own compromises, e.g. 2A3s are very lowered power, but supposedly sound great. KT88 are actually quite powerful and are capable of being used in 50watt amps, often sounding at least as punch as a good transistor amp. With regards to implementation, check on how groups like Wavac do things differently to other groups.
Hope that helps. Other forums that you might wish to look at for data include:
Zerogain, Hifiwigwam, Pinkfish, WorldAudioDesigns.
