Re: speaker power handling vs amp wattage
As long as the speakers sound the same, then it may boil down to if you want a little breakup or not. You will probably get a little more breakup from the 20 watt version.
It's not question of if the lower wattage version may blow up or not behind that amp, as the 20 watt, or even a 15 watt rated speaker, is still rather safe in that regard.
Another thing to consider is the speaker doping. Sometimes, a more severe application of doping is the difference between the ratings of two essentially identical speakers, but with different wattage ratings. Sometimes, with more doping you get more wattage handling. However, doping can effect the tone. Many people feel speakers with little or no doping sound warmer and more natural.
The higher wattage speaker may be using plastics and other more modern materials in the vopice coil design too? Perhaps not, but I'd be more inclined to try the lower wattage version myself.
As was well said, it's just a matter of which one sounds better to you.