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.