A higher power speaker can reproduce a signal more accurately.

think of a the sound of the song you are hearing
as you look at the spaker move in and out.
notice how it moves in an out to create the sound you hear.
A higher powered speaker tends to be more accurate
as to where the cone should be at any split second.
Because there is more power available, the accuracy goes up,
but also, the distance (throw) that the cone is able to move
in and out while still creating quality sound is increased,
giving the higherpowered speaker a wider range of
accurate sound production,
Now, the speaker that is moving in and out more, moves more air,
which is where the general idea of more wattage means more volume.

Lower powered speakers can oly move the speaker in and out a little,
and there is less extra power laying around to increase accuracy.
Less volume and less accuracy.

I can't think of a good analogy for this right now, but I will,
and I'll post it when I do. For now, Hope this helps.

If your amp is 75 watts x 4 chan at 4 ohm,
You could buy 1 of 2 things for about the same money...
A high quality set of 50 - 60 watt speakers or a lower quality set of 75 watt speakers.
I'd go with the 50 - 60 watt speakers. or I'd spend the moey for the better 75 watt sspeakers.

Now if you have a JL Audio 300/4 (75 x 4 at 4ohm) then I would go with JL speakers (all rated at 75 watts) get the biggest separates you can fit in your car (6.5" if you can)
Size matters. If you're gonna have subs, bass comes from the rear. a 6.5" speaker up front will compensate for that much more than a 5.25 will.

The other thing is I would get a nice big am 75- 100 watts x 4. put that in and see how it sounds.
then replace the front speakers and see how that sounds, then replace the rear speakers,
you may find no need to replace the rear speakers, leaving more budget for the fronts.