How many watts per channel are enough?

Both amplifiers will more than serve your requirements adequately.

Experience has shown me that WPC to not always serve as the ideal measure of power. Not to dismiss the spec out of hand, but it is an algebraic (steady state) measure, while we live in a world of calculus (dynamic).

Instead, I'll put forth the contrarian assertion that the ubiquitous transformer coupled push-pull tube amplifier with two tubes per channel will drive a loudspeaker far more similarly than differently in terms of seat of the pants power whether using 2A3 (10 - 15 wpc), 300B (20 - 25 wpc), or EL34/6550/KT88 (30 - 75 wpc). Doubling the number of output tubes to four shows the correlation still holds up. In other words, we will often draw surprising conclusions in regard to amplifier power if we move away from drinking by the label.

Your far safer using a bigger amp than the speakers can handle because now you have the luxury of using only SPLs to determine maximum speaker output, ie: pain threshold:)

Contrarian Assertion is the name of my old dude punk band.

Use nice, round figures...50,75,100.

I have a 34 WPC PrimaLuna Prologue 5 power amp. I have found that because of the ability of tube amps to distort musically, 34 watts of tube bower sounds more like 75 watts of solid state power. The amp will distort at or above 34 watts as rated but unlike transistors that hit a brick wall at their power rating and sound terrible above it (and can destroy your speakers), tube amps distort 'gracefully' and don't clip (or at least don't clip as abruptly).