But there is so much more to twinking then burst. That's hardly a portion of it.
For mutilate\PvE, I would take a 1.4 over either for more DP procs = more IP on mh.
For sub\pvp, then take BoU for the following reasons, imo:
-+22agi over crux, aside from the 44 AP, the extra crit rating is nice as well. Also dodge and whatnot. It complements your mh more then crux.
-I would still prefer a 1.4, but 1.6 is better for shivs then a 1.8