I think aLCuiN is saying for chest/pant/belt/shoulder items, it is better to have all resist option rather than armor option (so for other items you should not add all resist or armor because there are better options to add). use rubies for chest and pant sockets and rely on paragon points instead of diamonds for increasing defensive stats.
I am not 100% sure about the following 2 equations but as far as I know the equation for how much AR or armor reduces the damage is
current AR / (current AR + x)
current armor / (current armor + y)
I believe x is 300 and y is 3000 or something... I have to check again.
Anyways it means the more AR or armor you have the less efficient each AR or armor points will be at reducing damage. for str and dex classes since you get a lot of armor from primary stat, you should focus on increasing all resist. For int classes, if your all resists is over 2000, then you should consider adding some armor. However, that is only the case with the item affixes, for the gems I think you always want to use the gem that increases the primary stat.