It's very hard to compare the two performances. One is not a recreation of the other. Both actors are playing opposite ends of the same character. The hermit, "crazy old man" Obi-Wan is neither the same in personality or function in the story as was the case during the PT. The PT Obi-Wan was a well-respected General, with many like-minded people around him, at a time of relative comfort. I liked what Ewan initially did with him as a supporting character in TPM and the evolution he showed us by the time we got to ROTS. He's a very subtly written character but the development and little nuances, if you look for them, are a delight. By ANH, Obi-Wan is much more weary, broken and cynical than ever before, and therefore, Guinness and McGregor's jobs were very different. Both deserve high praise, though I feel McGregor deserves extra credit, not just making the character his own and equally memorable in a different way, but for impersonating and hinting so brilliantly at the many aspects of the man who played it before. There are times during ROTS where I can almost feel the two versions morphing into each other - the gravity and emotion in such a simple line as "Padme, I must find him" is particularly wonderful and chillingly close to Alec Guinness. That's all down to McGregor and the dedication and time he took to getting it right. By comparison, given Guinness' ability (I've no doubt he's the greater actor of the two btw) he was near-enough phoning it in during ANH. That's how good the guy was. By his standards, he could phone stuff in and still get nominated for an Academy Award.