For me, they are (or were) both great actors, but there is something masterful about Alec Guinness’s portrayal of George Smiley.

The film has the better supporting cast by far[0], but the original series is worth watching just for that portrayal. Watch his face as he slowly puts on his glasses during the beginning of the interrogation scene with Ricki Tarr for example.

[0] with the possible exception of another great performance by Ian Richardson

Edit: I very much enjoyed the film too, including Gary Oldman. But the miniseries captured the bleakness of the era, had time to explore more of the detail, and left more things implicit rather than explicit. All styles of production that I personally enjoy.

I watched the TV series some weeks or months after having seen the Gary Oldman movie, I enjoyed both, but the TV series is great while the movie I thought was just good. Both have excellent cast and are worth watching just for this reason, but the story with all the nuance seems better suited for the mini-series format than for a 2 hour movie.

I liked the scenery and set dressing of the new one greatly.

The old TV series had a superb ensemble cast. The new one in some ways let itself down. Hungary for Czechoslovakia. Turkey for Hong Kong. Peter Guillaim has a gay lover. Minor changes but also.. why?

I saw the movie with a friend who had never read the book nor seen the miniseries and he couldn't really follow it. I could (barely) from my experience from the book and series. Two hours is just too short to convey such a complex story.