They all have their pluses and minuses. Read Training and Racing with Power by Allena nd Coggan. They do a good job comparing. (and you'll want the book when you buy a Powermeter).
As for PT software, whatever meter you buy, get Cycling Peaks WKO. It will work withanyof the meters.So the olusy PT software, I don't see as a signficant point.
As for PT and water, this has gotten better. I never had any problem with my PT SL in the rain. I did manage to mess it up on the backof the car for 12 hours at 80 mph on the interstate in a heavy rain.
However PT replaced the torque tube (under warranty) with a new better sealed torque tube. If you buy a new PT SL or 2.4 I don't think water will be a significant issue.