If we assume the same speaker avg. load impedance (say 8 ohms) and similar current level amplifier, then 87db sensible speakers with 100W amp give the same results as 90db sensible speakers with 50W amp???
Not quite. It would depend on how sensible the sepakers really are. This would also only apply in theory and seldom manages to work out exactly in reality.
Always opt for the best quality sound. Watts alone don't make for superior sound quality, only decent components and a well matched system manage that feat. You won't have anything forever.
PK, if your coming from car stereo, you should learn that watts aren't all the same and are considered quite different when going from car stereo to home audio.