Well, in my limited experience, the 6 cyl AX15 tranny is the better one. The AX5 in the 4 cyl is not known for its strength. BUT, I am assuming that you are talking about a manual transmission. If you are talking auto trannys, then I really cant help you on that one. But, if you are talking manual, then you need to know that you will have to make changes to your transfer case the mate up the 6 cyl tranny. Different input shafts on the 4 and 6 cyl transfer cases. And, I believe that there is an extra transmission mount for the 6 cyl tranny.