John had a good link to a study in Ontario......Wish I kept it. 

Went to tracks in 2002.   Somethings gotta change in tires/ suspension before I'd go back to tires for tillage and planting.

PSI = pounds per square inch, that means in every direction from a single point there is pressure.  GRAVITATIONAL pressure has to be just a tad different.  Put 5 psi in your honda four wheeler, lay down and have your buddy drive over ya, id bet you'd be ok, put 5 psi in a 45,000 pound deere with tires and do the same,,,,,,,,,  And the tracks would be the same as tires on both, excet tracks on a honda 4 wheeler will get through absolutly anything compared to tires.

Both tires and tracks create compaction.  Tires create it over a wider path, its very obviouse if a guy takes a five mile drive in right after planting.

We rented a 9400 for a day last year while our quad was down with a broken hyd hose.......Our hired hand last year who was a 19 year old newbie green blooded #1 Jonny fan, was glad to be back on tracks. 

Hooking a mounted tile plow on the back of both tractors will give ya a good idea of why my opinions are what they are.

Thats all Im gonna say,  some guys will never switch, and they have valid reasons, just the same as mine for not wanting to go back.  

(carefull, sounds like there is a Mich tire salesman on this thread)

Good luck