I owned the A5 and I have listened extensively to the P5. Both are excellent multichannel amps, and sound similar. The difference mostly comes down to power. Whether you need the extra power depends upon the size of your room, the sensitivity of your speakers, and the volume at which you tend to listen.
I recommend you use a required power calculator to determine how much power you need.
I recommend you use a required power calculator to determine how much power you need.