Longer the better I recon.
If you are tracking the car and never putting it on the road then work the shorter end of the scale.
If you are mainly road with the odd track day then look at the long.
The majority of your road driving will be in the mid range, if you loose a little top end at the odd track day it wont kill you.
Jim do a dummy up.plumbers pvc tube,heat the end with a hot air gun then push it onto the small end of a funnel,it will flare.
Make 8 short, 8 long and see what you like,you can also work your hieght as well.
My 2cents worth.
jim