all right here goes im pulling the trigger on a set of kendas. is there a big difference between a 80/100x21 and a 90/100x21

 

10 mm in my book.
If you're riding it in the dirt 100% then go for the 90 profile, it'll allow you a longer footprint at lower pressure, giving more traction. If you're dual sporting then the 80 profile is the go. The 80 profile will offer more stability when cornering on the road due to its shorter sidewall.
What are you getting BTW, Trackmater II?