I still dont know why the speed upgrade for panthers was shot down. Im not saying in the original op2, or even standard in multi. But when you make a map and/or tech tree, and you include the tiger speed modification and tigers are FASTER than panters, thats just not cool.
The tiger speed modification was possibly more of an easter egg than anything. Furthermore it only applies to a few maps. It's not like every map has a wreckage for Tiger Speed Modification.
Thus in a general case panthers are faster. I think the main reason people don't use them is a general stigma that "Panthers suck" and they just go straight for tigers. You could make a sizable difference if you built panthers as soon as they were available.
Also, if you are in a case with very little rare ore (i.e. maybe just a 1-bar) then Panthers are more cost effective than Tigers. in other words, more bang for the buck.