It really depends on how you're getting your tone, if the amps are master volume, whether you're using a distortion/OD pedal... If you're getting most of your distorted tone from a pedal, it's not as critical. If you're used to driving the amp for distortion, a lower wattage amp will allow you to get the amp to break up at lower volume whereas a higher wattage head will give you more headroom but less distortion. Also keep in mind that a higher wattage head with a master volume will allow you to crank the preamp tubes into distortion at lower volume, but if you want the power tubes to break up, you're still going to have to crank the master volume up. IMO power tube distortion sounds smoother and more organic than preamp distortion on its own.