12" or 15"?
Depends on the execution.
I have a pair of speakers with two 12" bass units per channel in a large sealed box and a pair with a single 15" bass unit per channel in a large ported box. The speakers with the 12" bass units are about 93db/2.83v/1m efficient compared to 97db efficient for the 15". Neither of them need a lot of power to produce loud bass.
In terms of bass quality, I prefer the speakers with the 12" units. Crisper, more tuneful, more textured, more foot-tapping. The 15" ported speakers have a softer edge to percussive transients.
In my experience, the speakers are the most important component for getting punchy, crisp, kick-in-the-chest, bouncy, toe-tapping, tuneful bass. However, it is possible to compromise the quality of the bass if you have components higher up the chain with a woolly bass. If you have a decent digital source, with solid state amplification you should be fine as long as the speakers are up to it.
Is 40 watts enough - either RMS or peak? Depends on the speakers. For amp friendly speakers like mine, 40 watts is complete overkill. 1 watt is enough for loud punchy bass from them. For amplifier unfriendly speakers, you may well be looking at amps with more than 40 watts for best results.