Bill,
I have EI instruments in my Cherokee and several other folks on the field have both. I can't comment on which is superior. When in the Cardinal Flyers Group, the JPI generally got the nod as the recommended unit, but I don't remember any particular grounds other than they gave the biggest "group discount". I have talked to the folks at EI several times about various things and they have always been extremely patient and wiling to help with questions.
Oh....biggest reason to buy EI....they are in Bend, OR. You do know where that is don't you ;-)