Most (all?) of the magic items we've seen are more than just a flat 'make these numbers bigger' plus. So the balance between two items of different levels with the same/similar enhancement bonus effect can come from differences in that special power. For example:
Two items that occupy the Neck slot, and whose enchancement bonus applies to Fortitude, Reflex and Will defenses:
+1 Cloak of Resistance, Level 2
Power (Daily): Minor action. Gain resist all 5 until the start of your next turn.
+1 Amulet of Health, Level 3
Property: Gain resist poison 5.
The Amulet is constant, but somewhat situational. The cloak is very limited, to once per day, but will come in handy, guaranteed. Personally, I'd rather have the Cloak, which is lower level, but that might just be from my playstyle, and they are very close in level. Another thing to keep in mind is synergy. If the Dragonborn typically breathes poison and always finds he could get a lot more enemies if he could just include the damn fighter in the area, then the Amulet is better for the fighter in that group. Let's look at some other similar items with bigger differences in level. Here are two items that occupy the Waist slot and have no enhancement bonuses:
Belt of Vigor, Level 2
Property: You gain a +1 item bonus to the healing provided by your healing surges.
Ironskin Belt, Level 5
Power (Encounter): Minor Action. Gain resist weapon 5 until the end of your next turn.
These are both very appealing to any Defender, but the higher level item will almost certainly save more surges than the lower level item, while adding more in-combat survivability. Easy choice for most characters: Between the two, give me the Ironskin Belt. Even a wizard might prefer it: he probably only needs resist weapon 5 once per fight at most, but when he does, he really needs it.
