The entire Nephalem Valor concept has been removed since Patch 2.0.1 (live on Feb. 25, 2014). Along with this change, two new concepts have been added to the game (Official Patch Notes):
Pools of Reflection have been added to the game
- Pools of Reflection provide a 25% bonus to experience
- This bonus persists until you reach a set amount of bonus experience (equal to approximately 10% of the current amount of Paragon experience needed to reach the next Paragon level) or your character dies
- Gathering additional Pools of Reflection bonuses increases the amount of total experience bonus a character can receive until character death, stacking up to 10 times
- This experience bonus persists on logout
Nephalem Glory has been added to the game
- Occasionally during battle, monsters will drop Power Globes when damaged or killed
- Power Globes grant the Nephalem Glory buff, which causes players to deal additional damage in the form of golden lightning bolts and increases their movement speed for a short time
- This buff can stack up to three times
The formula in question seems to be:
DR = Ak/(1+Ak) where A = armor, k = 1/(50*moblevel)
So, for example, at level 5, with 100 armor, fighting level 5 mobs,
100*.004/(1+[100*.004]), or .4/1.4, or roughly 28%.
Fighting level 10 mobs, this changes to
100*.002/(1+[100*.004]), or .2/1.2, or roughly 16%.
Thus, when you gain a level, without increasing your armor, the character sheets displayed damage reduction shrinks slightly, since 'equal level mobs' are better able to penetrate your armor. Among other things, this means that there's no DR involved here, Armor just keeps getting better, the more of it you have.
Best Answer
In general, from about Normal difficulty Act 2 onwards, a +10% or higher bonus (from putting a Ruby into a socketed helmet) is going to beat the pants off of any +XP bonuses you can find.
The +XP bonus enchant tends to max out around +15 XP, which would mean it would beat a +10% XP enchant if monsters were worth less than 150 XP per kill. About midway through Act 2, I sampled some of the monsters XP values as I was killing them, by monitoring my current XP to level before and after encounters. Routinely, the average per-kill was more than 200 XP. As you progress through the game, the amount of XP per-kill will increase, and easily dwarf the small bonuses you can get from +XP gear.
This doesn't mean that you should not use +XP enchants on other equipment - but especially later in the game, it becomes less and less worth the investment. Almost always, if you have a choice between a socketed helm with a good quality ruby, or a non-socketed helm with a +XP enchant, take the helm with the ruby.
Bear in mind that the XP you gain from monsters also depends on your level versus the level of the monsters, and information about monster levels is not readily available in-game. Thus, the only real way you can say how much the monsters you are fighting are contributing to your XP is to hover over your XP bar before and after killing them, and then do the math based on these values.