RE: 2006 GO 170 vs 185
I think you might want to wait the 4 weeks.
At 90 Kg. /198 lbs. you can definitely use the extra 9 cm (3.5") of
width and if you intend to go try to race it the addtional sail size capacity
(10.5 m2 -12.5 m2) would make a significant difference if you also get a 70 cm fin to use with your largest rig.
The GO 170, at 90 cm wide, will work for you, very nicely, but not in < 10 - 11 knots of wind.
So, if you look at your conditions, how much < 10 knot sailing might you do? That's probably the most valid "decision point" here.
If you get 12 knots and higher, then the GO 170 with an =>8.5 m2 rig will be super.
Hope this helps,