Stat allocations are based on class, and are the same across all players.
Unlike Diablo II, there is no input required to assign stats on level-up. It's just free stats that happen the same for everyone!
As far as I'm aware, your character will always gain:
+3 Primary Stat
+2 Vitality
+1 to both Secondary Stats.
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
There are two ways to roll.
Either Tank/normal, or Glass Cannon.
In normal/tank mode, you want to stack both armour and resists (and preferably block chance if you're using a shield). You can use this handy guide to figure out how much armour 1 resist is worth at your current armour/resist levels. The reason people are stacking armour/resists and not just straight vitality is because of how damage reduction (DR) works. It works by increasing your effective hp (EHP). This means at 50% total DR, if you have 10k health, it's actually like you have 20k health, because a mob will have to do 20k unreduced damage to kill you.
Another benefit to having low actual hp, but high EHP, is that any heals are worth a lot more. If I have 100k health and no DR, and I heal for something that heals for 10k, I'm healing 1/10th of my total health. If I have 20k health, and 80% DR, even though I have the same EHP (100k), my heals are now worth half of my actual health, increasing the worth of heals (this doesn't work for any % based heals).
After DR, and some vitality, you want to stack enough dps stats to let you kill enemies before they enrage. These dps stats can be your main stat/crit%/crit damage/etc.
A few extra stats that help here (especially for Barb/Monk) are Life on Hit and Attack Speed %. Gear with these two stats can be worth a lot if they're a high amount, as attack speed % increases the effectiveness of Life on Hit, and Life on hit in high amounts (coupled with high EHP and lower actual HP) can be the difference between farming a difficulty and barely progressing.
For Glass Cannon mode you want to forgo any survivability stats (armour/resists/vit) and just stack DPS stats. This only works for the ranged classes (Wiz/WD/DH) because melee classes have to get close to fight, so they'll always get hit at least once.
This build works by having so much dps that you kill enemies before they can even hurt you. You also need to bring at least one survivability spell, because when you fight ranged enemies it can sometimes be tough to avoid their moves.
In this playstyle, all you need to do is shoot anything before it can get close. If it hasn't died in time, pop your survivability move and move away before shooting it again. You will be one shot by pretty much anything in Inferno with this build, but it's very easy to gear up for, so that's why people use it a lot, as opposed to the normal builds which cost a lot of money to gear up for.