I honestly feel that it is on a case by case basis! Oftentimes the first one to come out is the 'better' one; for instance, I am more fond of the Bleach manga than I ever was of the anime, but the Princess Tutu manga was a mess while the anime was wonderful. It would depend on each for certain! On the other hand, if you're just asking what I'm more likely to go for; it depends. When I'm deep into an rp I like to read manga so I can have it in one tab and work my way through it as I wait for replies. On the other hand, anime is great to watch with friends, and I like having anime to watch each season. (This season I am watching Gatchaman, the Eccentric Family, and Love Lab, which are all excellent-- and Genei Wo Kakeru, which is Not.)