If infinite person years, then (so long as life is net positive) we have infinite utility, and I can't see obviously whether doling this out to a 'smaller' or 'larger' set of people (although both will have same cardinality) will matter. But anyway, I don't think anyone really thinks we can wring infinite amounts of life out of the universe.
Total life-time will have some upper bound. So in worlds where we are efficiently filling up lifespan, the choice is between more short-lived people or fewer long-lived people. In the real world for the foreseeable future, that won't quite apply - plausibly, there will be chunks of lifetime that can only be got at by extending your life, and couldn't be had by a future person, so you doing so doesn't deprive anyone else. However, that ain't plausible for an entire society (or a large enough group) extending their lives. Limiting case: if everyone made themselves immortal, they could only add people by increases in carrying capacity.
If longer lived people tend to create more spaces to expand into in an infinite universe, and this results in reproduction at a normal or higher rate, that would indicate that longer lived people are more moral, since the disutility of the long lived people dying would be (relatively) absent from the equation.
If there is a point of diminishing returns on the creation of new people -- perhaps having a trillion lives is less than 1000 times as valuable (including in the sense of "justice") as having a billion lives in existence at a given time -- l...
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.