I am about to get one or the other.

What I want is:
- display of coordinates (as security feature, to call coastguard with waterproof phone and pass position)
- max speed in a session

Which one is best?

If you have a waterproof phone, then it is likely to be a smartphone... which already has a GPS - just use that. But if you are close enough to the shore to use a phone, there is also a pretty good chance you wont need GPS coordinates, as you could just speak to them directly. What you want here is a VHF radio or other sea-going lifeline.

As for max-speed, any will do if you dont care too much about accuracy. Neither the 201 or 205 are all that good compared to other products (when used for windsurfing).