I have the 6.0 and am 87kg and use the sail in 18 to 25kts on my 103l FSW board

I use my 6.0 Gator on my 90L Exocet Warp as well as my JP Xcite 110L. But I am only 70kg and can handle it up to 25kts at a stretch, so for your weight, I'd pick the 6.0 over the 5.5 easily. It's a really good sail!!
Cheers.

Hi. I am 90kgs.

I bought a 6.5, 5.7 and 5.3 this year. So far have had a lot of excellent sailing on my Fanatic Ray 115l with the 6.5 at Kyeemagh, Sanctuary Point and Canberra in around 20kts. The couple of times I have used the 5.7 it has been better with my "high wind" board an Fanatic Freewave 105l and generally around 25kts. Haven't been able to use the 5.3 yet. I got these sizes because they all fit on a 430 mast. Makes life easier.

I do like to be fully powered