Rereading the original text, I think he is talking about all three of (1) doing something that has a substantial impact on the world, (2) doing something that brings you major career success, and (3) doing something that turns you into a better scientist and a better person. (The last of those is mostly not very apparent in what he says, but there’s this: “I think it is very definitely worth the struggle to try and do first-class work because the truth is, the value is in the struggle more than it is in the result. The struggle to make something of yourself seems to be worthwhile in itself. The success and fame are sort of dividends, in my opinion.”)
Rereading the original text, I think he is talking about all three of (1) doing something that has a substantial impact on the world, (2) doing something that brings you major career success, and (3) doing something that turns you into a better scientist and a better person. (The last of those is mostly not very apparent in what he says, but there’s this: “I think it is very definitely worth the struggle to try and do first-class work because the truth is, the value is in the struggle more than it is in the result. The struggle to make something of yourself seems to be worthwhile in itself. The success and fame are sort of dividends, in my opinion.”)