Kite shield takes more hits than the knightly shields, and is a lot stronger than the elite cav shield, which is weak.  It has lower resistance, but significantly more hit points.  It is tougher...do the tests yourself!  I already have.  It also takes more axe hits.
The width advantage of the other shields is minimal imho.  You just turn a little to block a hit.  I've used all the shields and having just a couple points of width does nothing.
The larger round shields (large round, huscarl) have significantly more width, but they are slow.