The question, I believe, would need further specified to have a good or desired answer. We can clearly see that there are instances in which one can be better than the other. There total value would also vary according to certain variables, such as: What is the HP total of that character? Can the DR be bypassed? What is the rate of damage- per round, or per attack- that the character is taking?
Let us assume that the character has either DR 3/- or Fast healing 3 (as taking either of the epic feats would give a character exactly this, excluding other feats/abilities)
IF a character is taking 2 or more successful attacks per round, we can see that by the beginning of the first round, the HP of the creature with would be 3 higher than the FH (DR has prevented 6, where Fast Healing has given back 3). Which would show that DR is better in this situation, and likewise in any situation where the character would be taking more attacks per round (where the difference in benefit from DR over Fast Healing would increase even more). However, in a situation where the two given characters would be taking damage from one source per round, the benefit would be equal, but where the damage would be dealt less than once per round (he saves vs. a spell one round, creatures fail to hit his AC, etc.) than the benefit from Fast Healing will be greater (only 3 damage will have been prevented from DR, where the Fast Healing creature had the opportunity to regain 6 or more hp). Generally, the first type of situation is more of what I would expect from D&D encounters (takes damage from more than one source per round) so I would tend to think that DR would be more beneficial. Still, that is benefit in a single encounter. If, however, most encounters are spaced by a sufficient amount of time, and the character is not taking too much damage (as in they die before they get the chance to heal) in an encounter, than fast healing will do better. If DR is high enough, a character may not have to worry about taking damage at all. I would say that for a character (as concerned with epic feats) taking a small amount of fast healing, and higher DR would benefit him most. For a creature, whose purpose is for only 1 encounter (typically), DR will be much more beneficial (provided the DR and Fast Healing numbers are relatively close), since, as we see above, the will be taking more attacks/per round and will not have a great chance to take full advantage of Fast Healing as a PC would (between this and the next encounter)… although throwing in Fast Healing in combination with DR and good HP would make the creature quite enduring.