Light of Cosmos and Relic of Yu'lon I would imagine. Passive haste on Light makes it easier to reach haste breakpoints and the Int proc is nice, while the passive and proc Int from the Darkmoon Card are probably better than passive Int and a proc haste unless an even higher haste threshold for a temporary amount of time is better than DMC's Int.
Looking around at what other top raiding moonkins are doing, I'm seeing Light of Cosmos and Essence of Terror being the best. The uptime on Darkmoon card of the specific proc is rather short whereas the haste proc from Essence should put you into a new dot category gaining not only the benefit of all that extra dps but also far superior dots.
Not true theorycrafting here, but looking at Sunfyre's numbers for stats, intellect sits at 1.00 and haste sits at .52 after breakpoints. So that means is slightly better than half as good. Now looking at the trinket procs, you gain more than twice as much haste as you do intellect on Essence vs. Darkmoon card, plus it lasts 5 seconds longer. I'm sure the ICD on Darkmoon card is lower than Essence to compensate but you can see why strictly stat wise it's very competitive. But adding in the ability to hit new breakpoints pushes it's advantage further. Plus the higher stagnant intellect on Essence further pushes it forward.
I'll let someone do the true theorycrafting, but initial looks at the numbers and common sense make Essence better even on normal mode and certainly on Heroic. Darkmoon card should be better than Normal Light of Cosmos though, but not 100% sure of that. Definitely not better than Heroic Light either.
I am using Relic of Yu'lon and Light of Cosmos for over 1 month so I got opportunity to test them several times in raids. Based on my wol and other tests the uptime of Yu'lon is around 30% and for LoC 40%.
Don't have Essence of Terror, but based on wowhead comments it seems to have bigger CD then LoC (105 second ICD vs 45 ICD). Also I am biased why I would use EoT over Yu'lon or LoC because the proc is random and it is haste which goes lower to stats priority once u get to a haste breakpoint. On the other hand 6.9 k haste proc means that for 20s you will be probably on next breakpoint and if you get the proc at beginning of Lunar or during CDs probably would be a major dps increase ( more casts ). On the other hand if is procs at the beginning of solar I don't think that its more beneficial then the other 2. So its a little bit rng in my opinion.