I have always recommended replacing the factory headunit for better sound. Reason being an aftermarket headunit has a more powerful amplifier and also aftermarket headunits have much better equalizers, frequency response and tuning capabilities. basically the way I explain it is this, the aftermarket speakers can make the sound a tiny bit better but are still limited to the signal coming into them, where as the factory speakers with a great quality signal going to them will sound much better just from the higher quality signal alone. I hope I didn't confuse you by my analogy but I think if you ask other installers or people in the mobile audio business you should get the same answer, now replacing the speakers and the head unit is an even GREATER option that will surely give you better sound. Alot of people think distortion in a sound system comes from the speakers when in all actuality it is usually attributed to a lack of power from the Signal source, You are MORE likely to blow a speaker with to little power than to much power believe it or not, of course there are varibles involved but I will save that for a rainy day, or if someone wants more info. just pm me,
All expressions are my opinion and my opinion only, if you agree great, if you do not agree great, I will do my best to help anyone with audio related questions.