Definitely go alpine pdx.
The digital class amps run cooler, consume less amperage, and push constant power across different ohm loads.
I just redid my stereo with 2 Alpine pdx's.
That being said, if you want a Marine amp, go with teh PDX 5m.
I opted to go with a non-marine, and got the PDX-V9 for the same price as the 5m. The V9 is a beast, at 4x100 and 1x500.
Check ebay for new PDX amps. I think you can beat those prices you posted.