the question is a mid range char with bigger skill growth rate are lower than a high end char with lower skill growth rate?
I think Kensei is way better than Isshin in the long run, higher growth+base as well as his Bonds mean that he gains a noticable degree of survivability when going to PvE without VG, which you'll most likely do if you get Brenji anyway.
Since I have a tough competition on my server, I currently run Ichibe(->shini yoruichi, which I plan to replace with Senjumaru if priced reasonably or CSB kenny), kensei, main, hikifune and oetsu. Otherwise I'd most get Brenji with kensei and use Tenjiro instead of oetsu... which I could do anyway, but I just kinda like oetsu's failsafe mutiliate... and Tenjiro's bugged evo bugs me.