15watts can be enough, or not enough depending on how much clean volume you need. It really depend on how loud the rest of the band plays (especially drummers).
You can always use a pedal to get more grit from a clean amp, but you cant get more clean from an amp that is overdriving unless you turn it down and mic it up.
Its really a hard question to answer, but most of the dudes i know who gig regularly have two amps of different capacities and take along the best amp to suit the band and venue.
Wattage really pertains to low end response and clean headroom. Both of which go hand in hand. If you have a sound you like and you feel a little bit "undergunned" you can increase your volume appreciably by installing a more efficient speaker.
In the end, you are going to have to see if you can beg, borrow or steal one or more of these amps and see how they go either at gigs or in band rehearsals.
From what you have said, i get the feeling that a hot rod is probably your best bet because it has enough power for clean sounds in plenty, but also has two more footswitchable levels of gain. They are probably the most popular tube amp on the planet for good reason: they are good bang for buck and they give you everything most working players need. Yes they have their shortcomings (all amps do) but the main reason for hearing about them is because every second player out there has used them or owned them. They are a pretty standard, pretty humble but hardworking and versatile amp. If you cant manage to sound good with one of these, then you probably can't sound good using any amp.