HI,
simming is not my thing and installing SimC and AMrR only made thing even more confusing for me.
I am a Destro Warlock.
SImC values Crit over Haste, but AMrR vaklues Hastes higher.
When I use the BiB option AMrR always sugegsts 4 pieces T19 LFR, lets me keep ilvl 900 head and legendary shoulders.
My other items in these slots are at least 25 ilvls higher.
So whom should I trust? SimC with crit and higher ilvl, or AMrR with haste and T19 lfr 4p bonus?
Things are about to change significantly, within a couple of weeks it should* be automagic for you.
However currently to get the best advice possible you need to make a custom gearing strat for your character. There are a few guides around to follow to make one, depending on your computers speed it could take a few hours.
I think this is the place for you to start: Simulator Help – Ask Mr. Robot
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*assuming everything goes according to plan. 
A link to your character (or even better an export from the addon) would help us respond to this.
Also, the AMR simulator is generally more accurate for warlocks, so I’m not surprised you see differences.