I would say Mastery all the way.

Let me explain, with Hast yuo hit a bit faster with your auto attacks. As you say, the target will be in melee just a few sec.. and for those few sec hast will max give you 1 more white attack from all guys. so you might get 3 white instead of 2 white.. yeah ? Its also a mater of Killing machin procs which i tested out going max hast on my DK and i didnt really see it procing more often, I'm DW ofc. so i will have it procking on most of its hiden CD anyway.

With mastery you will in same sinario hit 2 instead of 3 white hits, but your FS hit harder for the hits you get on them then. And when they move out of range you might wanna finish off with HB and ofc you want that to hit harder then.

What i dont understand is why ppl go 2h when DW is more dps anyway ? As a DK you want to crit, since crits from a DK hit harder then all other classes. So DW will produse alot more crits throught killing machine then 2h's.

Any reason why so many use 2hs as boxers ? i solo played as DW frost for a long time.