Let's get down to business. Flat damage rating, generally speaking, is going to be better than crit dmg. Flat damage is multiplied by stats, multiplied by crits, and is always active for your abilities. Crit dmg, however, is not multiplied by stats, not multiplied by crits, and is a useless stat unless you are scoring critical hits. Crit damage does not benefit white hits at all.
For example, I'm going to use my 60 Cable as an example. I have 38.9% crit, max fighting for 25% and rank 5 energy for 15% bonuses to Energy Pulse.
13 Relics of Atlantis = 61 crit dmg
17 Relics of Xandar = 21 ranged dmg
.389*61=23.729 crit dmg
.611*21*1.25*1.15+.389*21*1.5*1.25*1.15=34.2975281 25 damage rating
As you can see, damage rating pulls ahead by a pretty decent amount because of its always active nature and being factored into crits as well.
If run the same numbers assuming 50% crit
.5x61 = 30.5 crit dmg
.5*21*1.25*1.15+.5*21*1.5*1.25*1.15= 37.734375 damage rating
For giggles let's kick it to 55% crit chance
.55x61 = 33.55 crit dmg
.45*21*1.25*1.15+.55*21*1.5*1.25*1.15= 38.4890625 damage rating
60%
.6x61 = 36.6 crit dmg
.4*21*1.25*1.15+.6*21*1.5*1.25*1.15= 36.526875 damage rating
As you can see, crit dmg does not become more valuable until I gain 60% crit chance on Cable, which isn't very realistic.
What this boils down to is that unless you can reach extremely high crit chance, maintain a high uptime on FFoF, or you're a DoT heavy build (DoTs are currently doing twice their coefficient on criticals [ex. a DoT that normally does .25 dmg per tick does .5 crit dmg per tick]) flat damage rating is going to be superior. For most people, a Doc Doom/MODOK/Doc Oc with Edge, Metasensory, and damage rating affixes on your costume will net you the biggest DPS return. Again, this DOES NOT apply to FFoF or DoT heavy heroes.
Edit: To further reinforce flat damage, if you hover over your crit rating in game that is your chance to crit a target of the same level. At level 60 bosses are level 63, so your crit chance is going to be lower than what your paperdoll says.