Germination also has the issue that the extra Rejuvs do not interact with some talents and artifact traits as if they were 2 seperate Rejuvs. For example, Dreamwalker will only heal a target with 2xRejuvs twice. If you take Cultivation, you do not get 2 Cultivation stacks/bonus Cultivation healing from having 2 Rejuvs on a target. These factors actually reduce the value of having the second Rejuv over having an additional Rejuv on a unique target - probably to the point that they offset the mastery gain from having the second Rejuv.
It's just heavily dependent on damage patterns. Mathematically, you would need to have something like double Rejuv on 6-8 different targets at a time before the throughput gain from the extra mastery healing outweighs the gain from casting 1 CW every 30 seconds. CW is an extremely high throughput/high HPM cast, so it is definitely - mathematically at least - much closer to taking Germination in value than a lot of people think. It's just going to be all about whether you need to have double Rejuvs on targets or not and how often/how many of these you practically run with during a fight. If you want to focus heal a handful of targets at a time, Germination will win. If you want to kind of HoT blanket and AoE heal, CW will give you more value. If you run Cultivation, Germination is less valuable. I don't know that Prosperity is going to add enough value to pull ahead CW or Germination - even with 4 piece and SoTF. That's definitely the only situation where it's even a viable option, but I just don't know that even then it adds enough to lose the extra play style options CW and Germ give you.