The only difference is that you get an external I/O box. I think I recall reading the ADC might be a bit better too with the Platinum Pro. So if you're mad into recording & really need all those extra connection you should go Pro, if not just get the Platinum or a regular ZS.
Then again, according to 3DSoundsurge's review the Audigy 4 Pro is a better card for recording with than the 2 ZS Platinum/Platinum Pro. So the way I see it if those connections & recording capabilities are essential a Audigy 4 would be best, otherwise just a plain Audigy 2 ZS.