I would say for most general usage a superior kite shield (ala relic shield) will be the best way to go. Just remember, besides stats there is absolutely no point in having a shield unless it is actually doing its job and blocking. I've found tower shields to be terrible at this on a normal basis. Then again circle shields block very well but the mitigation is no where near as high. The difference between the two is that your circle shield will no doubt add to your overall average mitigation and in turn make you a tad less spikey from damage incoming than say a tower shield will. The addition of Bulwark changes things slightly though. With the additional 60% block rate, all of a sudden your Tower Shield makes you very durable for 15 seconds, at least from physical/blockable damage. I can see carrying a Tower Shield for certain bosses just to block certain things with that high block power and bulwark to come in handy, I just haven't seen a situation like that yet.