With Ed TV. Although fundamentally different to anyone who's seen both, the two were widely compared at the time due to their close release dates and (at the time, very unique) plots about a guy whose life is turned into a television show. The main difference: Truman's was set up before he was born, with actors playing the roles of family and friends, and Truman didn't know about any of it. Ed's was set up in his thirties (at his volunteering), involved all his real friends and family, and was completely in the open. Both were visibly prescient about (and arguably semi-responsible for) the forthcoming rise of reality television, with Truman being the groundbreaking speculative conclusion and Ed the more true-to-life concept.... for now.
This film also dueled with Pleasantville. Both movies presented idealized, televised (or from television) worlds juxtaposed against the cynicism of "reality", with its characters coming to the realization that they would trade their utopia for a more fulfilling, expansive world, even if it's not perfect.
What Could Have Been: Though the original script contains many scenes that made it to the final movie, its tone was much Darker and Edgier, reading more like a sci-fi thriller and set in (a fake version of) New York — for example, the scene where Truman threatens Meryl is much more violent, to the point where he actually tries to drown her in a sliced-open waterbed. He hires a prostitute to dress up as Sylvia. He also threatens to kill a baby at one point. (All things considered, probably for the best that it was altered.)
Truman was originally supposed to be played by Gary Oldman, and Christof by Dennis Hopper (Ed Harris was the last-minute replacement).
Truman was also supposed to be just out of high school, but since Carrey was in his thirties, it got swapped from teenaged angst to more of a midlife crisis.
An in-show example in a deleted scene reveals that had Truman not realized what was going on, Christof and network execs were going to broadcast the main show and its Spin-Off on a two-channel format; the main show following Truman and the spin-off following his unborn child, repeating the cycle all over again.
A scene in the original script depicts a staged rape scene witnessed by Truman, who doesn't go to help the actress about to be violated and just moves on. When he's gone, the actors and actress return to normal and express wonder at how he didn't try to help, or even do anything about it.
Several deleted scenes make it clear that Louis (Marlon) truly does care about Truman and gives him a moment of redemption where he finds him during the search and lets him go.
Word of God: Extra material offered by the writers reveals that Christof was an acclaimed filmmaker who won an Oscar in his 20s for a documentary on the homeless. He created The Truman Show soon after. This emphasizes what he's trying to say in the main page "What the Hell, Hero?" quote — he really has tried to give Truman the perfect All-American life, sheltered from the cruelty and absurdity of the real world.