Not a team member, but hope I can help
decisions are always hard. Do I make the right choice or not?
In your case I would go for The Go 151. why?
You have more choices in what kind of level you want to surf and for the wind you mention the 171 is very big (16-21kts)
If you want to go with the modern kind off surfing (shorter and wider)
Just let go of your old know-how. It is new. and you get used to the new boards very quickly. Even your boy who doesn't know anything about surfing.
My kids went on the board and after 1 hour they where surfing on a Futura 133 with a 2,5m sail.
( 9 and 13 years old).
If you change the fins ( as mentioned in the productpage) you can easely adjust your level
I don't think you will use the daggerboard from the GO windsurfer after a short period.
hope this helps