Besides diameter and strength differences what other differences are there and when would you choose on over the other? 5/16 is stock, right? Does anything need to be modified to run 3/8?

 

5/16" is stock. When one gets into large spring pressures, the pushrods could bend causing loss in valve lift. I would assume that mostly solid roller applications would worry about this, but if you had a very stout hydro spring, it could come into play. The pushrod holes on the heads would need to be enlarged to fit the 3/8 PR. The new ET heads are already enlarged.

 

Push Rod Info