Of course it's a priority system once the fight starts but I think Ryuko has a point. It definitely depends on the fight mechanics, gear and your skill to time Festers and manage everything else.

Just for fun I did some calculations just very roughly. Assuming all the dots run their 'full' duration (base+contagion) the potencies are:
Bio 440, Miasma 455, Bio II 525. That means the base damage from dots is 1420. Add in RS and that makes it 1704 potency, a 284 potency increase.
Compared with what Ryuko said about Fester critting, assuming RS and all dots are up for both Festers and Fester crits both times that's 450 potency each compared with 300 normally. That's an overall 300 potency increase. Of course there's all the other factors that play in but unless I made a mistake, and please excuse me if I did (it's 6am) the difference in damage should be negligible at most.