All people should keep in mind that using to much haste will bring you near to the gcd.
I think in most cases where the difference is nearly null you should go for spellpower.
If you use this calculator
Spellhastecap Calculator you can see how far away you are from the cap.
Indeed some people say, that you have to use potions or icy veins not in bloodlust, but in some fights you have only a small timerange where you couldn't use icy veins and bloodlust in a series.
And there is still the lag problem. If you have very short casttime (nearyl 1sec) the lag factor becomes bigger.
At last there are fights where the enrage phase is a phase where some pc systems become very busy and cause more lag.