I just be happy to maintain my polybian armies without the marian if I can... The popularis usually occurs when you least thinking about it... Polybian Princeps are better and more manageable anyway... (this especially true when you going to fought a lot of naked dudes or drapanais...) the princeps can hold better than cohors reformata... I know they only had 80 compared to 100, but they matters in getting heroic victory and building stars...
It really depends actually. If you can get cohortes reformatae up to 2-3 chevrons, they actually hold up pretty well compared to polybian principes. And then there's the added benefit that you can recruit them almost anywhere in the mediterranean... -M