I know when they were both fully alive, Yoda was probably the more powerful Jedi.
But surely Darth Vader new how powerful Yoda was. And right before Vader killed Obi-Wan-Kenobi, Obi-Wan said, “If you strike me down now I will become more powerful than you can possibly imagine.” So that sounds like after he was killed he became more powerful than Yoda?
But then Yoda eventually died, did Yoda then become more powerful? or does it depend upon how you die? It did seem a dead Obi-Wan made far more appearances than a dead Yoda…so maybe Obi-Wan in the end was the more powerful Jedi…though a quasi dead one.
I think that this is one of the things that is supposed to be addressed in Episode III. IIRC, the idea is that Qui-Gon supposedly finds/found a way for Jedi to be useful after they die. Remember in Episode II you do hear his voice after Ankin kills all those Sand Raiders.
I think it’s a mistake to assume that Vader understands how powerful Yoda is. He might have understood, before his corruption, but I think that he lost some in that respect when he went over to the Dark Side.
As for Obi-Wan manifesting more, that’s just because he died earlier. I mean, why the heck would Yoda manifest during Empire? He was still alive!
Knead is on the right track here. Yoda has definitely got more Force mojo, witness their fights with Darth Tyrannus. But as a martyr and motivator for Luke, with the ability to appear to Luke, he actually undoes everything that Vader may have worked for. And he knows it.
As for the Dark Side being more forcible, I also think that is unquestionably the case. The Light Side is not about sheer power, but understanding the interrelatedness of everything.