If it's one of the M&M games and you can harden the item so that it doesn't break, take the one that gives the highest total AC. If an item becomes "broken", I understand that you lose any magical bonus, but you retain the inherent AC of the item. Under those circumstances, it may be worth thinking about the one with the better basic AC, but on balance, the higher total AC will still be probably better
I must confess that I seldom use items with an AC boost. My favourite bonuses are those with Resistance pluses, especially "of the Phoenix" and "of Protection", closely followed by the items which regenerate hit points or mana. Also, I generally try to keep a few items "of Sanity" for use in Castle Darkmoor. I get sick and tired of curing insane characters (pun fully intended ).
I always trade in items which boost stats unless I'm lucky enough to find a decent one when my character is still at a low level. When a character gets to a high level, the "break points" in the "stats vs skills" table are so far apart that the benefits that accrue from using these items are vanishingly small at best. It is quite possible to wear an item "of the Gods" and gain no benefit whatsoever from it. You may need them for other reasons, of course, like gaining skill points from the pedestals in MM7.