I use the matrix cube for PVE just to get my str and crit up more. So depending on your gear, stats, etc., you cannot go wrong with the dual WH power relics in PVP or PVE.
Here are some number crunches I did about a month ago for my guild because I too had the same question.
First, here is my spec at the time of the test (I may have changed a few points around since this test):
Watchman
Stats and results (Each test was 5 minutes and best 1 out of 3 for each test):
1 WH relic, 1 Matrix cube , Exotech Might Stim:
STR: 2151
DMG Primary: 979-1141
DMG secondary: 202-303
Bonus DMG: 655.1 (842 power)
Accuracy: 102.89% (285)
Crit Chance: 35.37% (307)
Crit Multipier/Surge: 75.82% (285)
Total DMG dealt: 511,112 ( DPS 1,676.8 )
Total Healing done: 85,797 (HPS 281.5)
2 WH relics, Exotech Might Stim:
STR: 2063
DMG Primary: 992-1154
DMG secondary: 202-303
Bonus DMG: 668.3 (921 power)
Accuracy: 102.89% (285)
Crit Chance: 33.55% (280)
Crit Multipier/Surge: 75.82% (285)
Total DMG dealt 491,832 ( DPS 1,614.2 )
Total Healing done 83,285 (HPS 273.3)
The last relic combo I tried was with the DMG proc. Those three tests resulted in lower numbers than the above tests, so I won't bother posting those results. And for those that are curious the DMG proc relic averaged 6k dmg over all three tests.
I also, for <censored> and giggles, tried out a PVE healing relic. Yes, burn heals proc this relic. No it did not add very much to overall healing. If the affect was not limited to every 20 seconds (iirc), it might be worth it in some situations.