Minecraft

Personally I'll never use anything but FTB for the forseeable future. I care nothing for the politics behind any of it, I just know that the majority of the developers of the actual mods play on the Forge server and interact with each other, actively working together, fixing bugs, tossing out ideas, etc. It is all well and good that Bukkit might be actively updating and keeping abreast of changes, but when you can actually hear the mod developer talking to the Forge people live on a stream and working out a bug right then and there, that seems like the best arrangement you can have.

I think Bukkit servers dominate simply because that is all anyone ever had previously, and a lot of them probably don't want to change, most likely destroying their worlds in the process. Honestly I see Bukkit fading into obscurity eventually (although it will take a few years). They waited far too long to actually get on board with all the big changes, and as a direct result of that Forge was created. Devs that were anxious to update their mods had nowhere else to go, and the Forge server/people made it fairly easy to jump on board and feel like they were part of something bigger (i.e. not just focused on their own mod), so it was a win for everyone involved pretty much. I'm not a dev, but it has to be pretty cool to be testing your mod out, see that it conflicts with another mod, and be able to work it out right then and there with the other developer. Or even have a third developer suggest something that makes yours even better/easier to code. Or have people watch the live streams or Direwolf20's episodes and catch a beta version of your mod and express how much they can't wait to see it, or what they might want different, etc. I don't think Bukkit has that sort of visibility (I could be wrong though), and FTB/Forge is the "cool kid" so it will be really difficult to overcome that hurdle if they want to stay relevant.