40 watt would definatly do it, if you need more volume then like 1bassleft said you can always go through the PA, good thing about lower wattage combos is you can realy cook the valves, before we go on stage my amp would be warming up on standby for a good hour, when we get on stage and start playing the valves just sound better and better, one bit of advice, when you finnish your set switch your amp off and try to leave it undisturbed for 20 mins, give it time to cool down, you could run the risk of damaging your preamp and output valves.

I go with Lee, but there are a lot of variables. Small-medium in the UK means 100-200 people – is this the same for you? Does the audience only hear your amp,with no PA micing? Remember, an 80W amp is not “twice” as loud as a 40W amp. The ear doesn’t work like that. But an 80W amp won’t have to “work” so hard, esp with another 12″ shifting a bit more air.

If you like a bit of ‘crunch’ for your style, the 40W (maybe + extension cab) may be what you want. OTOH, the 80W with a good pedal could do it for you. As Lee says, more info on the brands you’re looking at would help, as would the likely gigging.

i had an AC30 33watt twin speaker, i played small to medium size venues and i could have done with a bit more headroom on the clean side, with a 40watt amp you could play those venues, but i’d say go with the 80watt for the clean, especialy in a rock band.
just out of interest, what models of amp are they? Ashdown?