I think I can suggest a few that beat either. Asimov's own Last Question, for one. Or try the Baxter stories involving the Downstreamers, or for early SF Stapledon's Last and First Men or, even more so, Star Maker.My favorite author is most likely Philip K Dick, but I really appreciate Isaac Asimov for his everyday approach to technology, which makes reading his science fiction works feel like reading non-fiction. Also, the incredible chronological scope from Robots to the Foundation series. To me only Robert Jordan's Wheel of Time comes close in terms of sheer time-span.
There are quite a lot of stories set in the extremely far future, but most of them don't take us on the journey so IMHO they don't count.