A KO'd player does zero DPS. Even if they are rubbing their face across the keyboard it is still more DPS than if they are down. As an RDM you definitely don't have to prioritize rezzes over your own DPS, as a healer though, leaving KO'd players down is directly neglecting your primary duty. It's understandable if you need to dodge and swift is on CD, but learning a fight so you know how to hard cast a rezz is one thing that separates the good healers from the rest. A BLM main taking on the healing role will have a distinct advantage here.
Outside high-end content, there is no priority. The priority is dictated by the situation. This is one of those things that is taught in healing 101 for RDMs, and tested with a situational question:I'd say the only big difference I find with raising as a RDM versus raising as a healer that I see would be my raise priority. As a healer in anything below Savage I'll raise dps first, tanks second and a cohealer last. This is because that should be the most damage efficient order. As a RDM/SMN I'll raise healers quicker since I'm less confident in random healers being able to solo heal anything for any length of time.
"One tank is down, and the other is near critical. One of the healers is also down. A DPS player just messed up mechanic; leaving a vuln stack + crit condition on both him and the other healer. Unavoidable raid AoE is coming that they will not survive. Raid AoE goes off, and the second healer and DPS player also drop. You can only raise one of them as your current resources are limited."
Who do you raise?
A. The tank B. The healer that was down C. The healer that just died D. The DPS player E. ball up in corner
Since both healers are down, it is pretty clear you have to raise one of them. However, if you raised the healer that just died, it would have literally been better to choose any other option; even E. The reason is because the healer that just died very likely does not have the same resources available to them as the healer who has been down; and in this clusterfork of a situation happening here, there isn't a whole lot healer C can do since they can't even heal themselves out of critical condition, and has to hope the winds of fortune blow while they replenish their resources.
tl;dr? Evaluate the situation before firing off a rezz as any job capable of doing so. That is part of the fun with rezz duty, and why I recommend to leave it to the healers unless absolutely necessary.