Wizards' Handbook by Dictum Mortuum (DnD Optimized Character Build)/Tricks and Comparisons
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 Items that grant a bonus to AC versus a defending weapon
And by items that grant bonuses to AC, i am referring, of course, to ring of protection DMG, amulet of natural armor DMG, and all those that cost bonus squared * 2000 gp. The main idea is to use a defending DMG, and by pumping its enhancement bonus via magic weapon, greater. Then you will be able to transform the weapon's enhancement bonus to dodge AC and benefit. Some DMs may have a problem with this however. Ask your DM if he is ok with this. See these graphs below for detailed information:
This shows how price scales between the two items. The defending weapon has a fixed price, but requires the expenditure of a spell each day. Ring of protection on the other hand, requires nothing to be spent on a daily basis, but its price becomes very expensive when reaching higher AC bonuses. If the spell per day is a problem, buy a pearl of power DMG for the lowly price of 9000 gp.
The idea of item levels is introduced in the MIC. This shows the level you will be able to afford the item and the bonus it will probably offer. As you can see at early levels ring of protection can be acquired easier, but at mid levels you will receive more benefits from the defending weapon. At high levels the ring of protection can be acquired earlier, but the cost will be too high.