Hmmm, think a lot of people are overlooking the simple math part of this question.
ALL new mobs hit for over 500 and most can double, triple, or quad hit per round. 1% shielding = 5hp saved from your hps PER SWING. More realistically, the mobs in high end zones hit for a minimum of 1k and some up to over 2k per hit.
AC doesn't do much if the mob has high enough attack, like those in Tavci. I haven't played a tank in a long, long time, I'm not sure of the formula of what AC has on a per swing basis, but I'm sure 20-30AC won't reduce every hit by 1%. On smaller mobs with less atk, AC really comes into play and can make a huge difference in the damage you take.
50 HPs just doesn't stack up against AC or Shielding. MAYBE if you were a caster since their ac is generally so low, but certainly not for a tank or anyone who gets aggro a lot.
Now, with all that in mind, it totally depends on your class and play-style. If your shielding isn't maxed or near max, shielding owns all imo IF you're fighting higher end. If not, I'd go AC. IIRC, AC also will help in the calculations of avoiding getting stunned (overall). As far as just comparing the shielding, look at it like this. If you have 7500 hps (most have at least that buffed), 1% shielding will allow you to take 75 more points damage before you die if you don't get a single heal.