The AR ratio is the relationship b/w the area at the entrace of the housing to the radius from the wheel centerline, as the housing scrolls into the center this ratio remains pretty constant.
A smaller AR increases spool up as it increases velocity going on to the turbine wheel. But the tradeoff is it becomes more of a flow restriction at higher RPM/more flow.
A larger AR will spool slower, as the velocity is less at lower revs and will take more gas to flow enough to get the wheel turning, but it will have less backpressure and make more power.
If you can deal with the slightly slower spool the larger AR is better as the setup will be more efficient and make more power per psi of boost and wont be losing it due to extra heat a backpressure from the smaller AR.
91 GT 302 CID c4 w/brake
TFS TW heads, f-cam, typhoon
70mm masterpower single turbo setup.