When im healing Arena with my Magicka NB i prefer Mutagen. because when one really get hard damage mutagen push him up and our hots (funnel etc) have more time to do there job...
Mutagen is an incrediblt cheap oh crap heal, the burst you get when applying on low health target is always bigger than the tooltip once you have the passive that increases low health healing. Effectively you push the target from critical danger I.e. from below 20% to over 50% health and get full duration of the HoT with same cost.
I like the dual purpose Mutagen (wich is a basic HoT and a front loaded HoT on low hp targets) much more on non-templar healers, where on templar I use rr. Only reason to choose otherwise for me would be the nightflame set, as you'd want as many heals as possible ticking over. But no, it's no consensus, just opinions.