WinAmp has a higher volume output than PowerAmp. PowerAmp has an unacceptably low volume output. That's why I don't use PowerAmp. I tried many, many music player apps, and PowerAmp is the only one that outputs a lower volume signal. If you try to adjust the master volume in the equalizer settings of PowerAmp in order to make it as loud as other music player apps, the sound will distort.
My phone (LG Optimus) has a low volume output through the headphone jack so I could not use PowerAmp and listen to music at a reasonable volume through the headphone jack-- not even if I plugged it into my car stereo and turned it up full blast. (My hearing is normal.)
Besides, WinAmp has a few features that I think make it a better music player app, such as Itnernet Radio, LastFM scrobbling, syncing with your PC, a plug-in app (MusixMatch) which displays lyrics, etc.
You can always try both. PowerAmp has a free trial. You may be able to overcome the low-volume bug by trying to disable Direct Volume Control in PowerAMP Settings => Audio Engine => Advanced Tweaks.