The most important issue here would be "Do you use the centerboard on your 2007 Rio L"?
If you are sailing without the centerboard down and are able to stay upwind and get back to where
you launched from, then a Go 175 should work pretty good.
If you only sail in very light winds, with an 8.5 m2 rig (have you considered a larger rig?) and are
not planing, then a centerboard ould seem to be a very good thing for "cruising" in non-planing conditions.
Either board is good for the moves you are suggesting you want to learn, but for water starts and
tail first sailing, the GO 175 would seem preferable.
Realise that moving from the very wide '07 Rio L to the new Rio M or the GO 175 will lose you some
width and for a big guy, width (and the stability it gives you) can be pretty important.
Hope this helps,