Which stats to choose and why???

When choosing a stat for your class such as haste, crit, mastery etc their are a few things to keep in mind that make it simple and a few ways.

1. First thing you should do is to check Claywarlock (here), Ask Mr. Robot, Noxxic, Icy-Veins or whatever you prefer to see how they spec, generally they will be identical with few exceptions.
2. You should know the value of your stats, for all specs listed on Claywarlock i post how many points give you 1% so you know how easy it is for that 1% to be obtained. In short with no special bonuses 1% Crit or 1% Haste = 1% dps/hps increase, special mana gain bonuses, dps procs, etc change that but its a good start.
3. Play close attention to your types of spells, dot's and hot's hints at haste, extra ticks can be potentially better then your best stat (crit/mastery)
4. Never think you know it all, once you think you know it all you stop searching and you stop improving, something may change or their may be something you don't even know yet.