It isn't really an either/or thing, because there is no cost to a failed listing on the auction house. If the auction ends without any bids, you get the item back and can sell it to a vendor. So the only limitation is that you can only have 10 active auctions at a time, and you should consider using the auction house right away. You will make a profit anytime that you can auction for more than vendor price +15%, and as long as you never set your price lower than that, you can't lose money by trying to auction something.
My guess is that there won't be much of a market for magic (blue) items on the auction house, since rare (yellow) items are significantly better and will remain pretty cheap since they can be resold when you upgrade (unlike in WoW). So in practice, attempting to sell blue items will probably not get you anything, and you'll just end up vendoring it anyways, but unless you hit the 10 auction cap or needed the money right away, you didn't lose anything by trying. And there might be some blue items which can auction for slightly above vendor price + 15%.
Strength increases your Armor by 1 point per point.
Dexterity increases your dodge chance by 0.01%-0.1% per point.
Intelligence increases all resistances by .1 per point.
This has a number of interesting effects - most notably, because of the way Resistance works (it's functionally identical to Armor), stacking small amounts of Int on a Barbarian, Demon Hunter, or Monk can result in a dramatic survivability increase.
Resistance, like Armor, will reduce all damage you take, and, like armor, you gain substantially larger marginal benefit by increasing it when you don't have much. Since typically, a Monk (for example), will have a reasonably high Armor stat from gear and passives, and very low Int, adding just a little bit of Int via an oddly itemized piece of gear can provide a really large survivability boost - certainly more of one than trying to add Armor via Strength.
However, because everyone has a decent amount of Armor inherent in their items, increasing Strength for a similar benefit is generally not worthwhile unless you're playing a Barbarian that wants the damage boost as well, or a Wizard or Witch Doctor with a very high Resistance stat already.
Best Answer
From the description of the skill.
I believe the bonus abilities work the same way yours do.
This can be altered using runes, however.
Keep in mind that while the skills runes give you are similar in behavior to your skills, they do different amounts of damage.
Only thing I'm unsure of is how much health/armor/resistances/MF/etc each ancient has. An ancient can die, but I haven't been able to find any information on how much health they have. I assume their health/armor/resistance stats match yours unless anyone can prove otherwise or Blizzard comes out and explicitly says.