Gemming

With lfr/flex gear should I get the socket bonus or just gem pure haste?

Assuming you've hit 7.5% hit and 15% expertise, yes.

It depends on a few things:

If it's a red socket and you're currently reforging into expertise, it's probably best to put a exp/haste gem there and get the bonus.

If it's a blue socket, it then depends on how much hit you have. Again, it you're reforging into it, gem haste/hit. If you're reforging out of hit, then consider whether using a haste/stamina gem is worth the bonus.

Stamina bonuses are great (especially on the helm, 270 for one gem), strength and haste bonuses are ok, dodge bonuses are kinda weak, and crit bonuses are irrelevant.

Try using a reforge addon or website to do the math for you. I use Ask Mr Robot myself.

Yellow and Prismatic: 320Haste of course.

Red: As said, if you're still reforging towards Expertise, or have a lot of items where you are not reforging away from Expertise yet, then there's no reason NOT to use 160Haste/160Expertise, so use it. If you're already reforging away from Expertise on nearly all items, then it's quite possibly a loss to gem for more Expertise. The less reforge options there are to get to the cap, the more chance that you're overcapping with a large amount. If the socket bonus is worth it (Large amounts of Stamina or Haste), gem 160Haste/160Strength. Otherwise, gem 320Haste.

Blue: See the comments on Red, but for Hit instead. However, since we only need half the amount of Hit, it's likely you're already capped while reforging away from Hit on nearly every item. Stamina is a better option for us than Strength, though, so if the socket bonus is worth it (Small amounts of Stamina, Haste or maybe Expertise), gem 160Haste/160Stamina. Otherwise, gem 320Haste.