Here's the number of melee damage the pet did for each build, to see the difference.
Melee - Damage - Difference - Difference %
Haste - - - -325 - - 916825 - - 253599 - - - - - 6.2
Mastery - - -306 - - 863226
Mastery pet dps: 4282.60
haste pet dps: 4668.64
difference: 386.04
difference %: 9.01
Just to point something obvious out here. Haste has absolutely no effect on Claw, since it has a 3 second cooldown, and since WH has a 100% uptime, focus for your pet? No issue. Also, you might notice that there is a 386 DPS difference on the Pet, but on the Hunter itself it was only 207? That's because as Mastery, your pet did 4328.25 DPS and the Hunter did 33435.95 DPS. In the haste build, the pet did 4707.38 DPS, the Hunter did 33208.02 DPS. (
Note: The DPS numbers here aren't from the exact same sim results as the ones above. I believe I had a low Kill Shot range % in the first results at the start of my post.)
As you can see, the hunter itself has a 200~ DPS
decrease by going to the haste buid, while his pet has a 400~ DPS
increase.
My conclusion: Remember that this is with max pet uptime. This can be unreliable (Hagara, Spine, Madness), so I wouldn't count on your pets DPS to give you an overall increase. Also, with AoE in the picture, I think it's safe to say that Mastery will usually pull ahead due to higher (s)Explosive Trap damage, as well as higher (imp)SrS damage. Personally, I would say that with the info above in mind, Mastery is the way to go for most of the current content.
Do note though, that this is without 4cp, it's for my Hunter, and the result can be different on your Hunter.
Also, both ways are perfectly viable. 200DPS will hardly matter.