What a great question! I might have to use this comparison the next time I teach this novel. Well, you are right in thinking that there is a lot to compare between these two texts. Firstly, both Winston Smith and Truman are in a world where pretty much there every move is observed and watched. They have no privacy whatsoever and their exterior lives are completely controlled by forces beyond their power. The only true escape they are able to engineer is interior: in their thoughts and minds.
Of course, a massive difference is that Truman at the start of the film does not know that he is under constant surveillance, and at least the powers that control his life are not malevolent in the same way that Big Brother is for Winston Smith. Both main characters try to escape or rebel against the powers that constrain them, but only Truman's rebellion is successful. 1984 offers critical comment on totalitarian regimes, whereas The Truman Show comments upon entertainment in today's world, the modern surveillance society and reality shows. Both, however, and this is the point I guess, show the consequences of limiting the freedom and the privacy of the individual.