Sadly, as another book was published on Swift after I'd written about 60,000 words of a 100,000 word biography, I never got the chance to publish my Swift biography. It was to be written to the same level of detail as my Mercer biog. ah well!
To answer the question though.... The majority of people who saw both Swift and Trautmann at their peak rated Swift the better keeper. Sadly time moves on and there are few alive now who can honestly say they saw Swift at his peak - his final post war seasons he was rated the greatest keeper in Eurooe but had become more of a showman than a great keeper towards the end. Also, it's worth saying that many Blues today claim Colin Bell as the greatest but how long before David Silva or someone else is perceived as the greatest ever player? Will the older generation accept that? It's all subjective and difficult to compare eras. I once interviewed Johnny Williamson who did actually play with both and he said "you pick which one you want and I'll have the other because they were the two greatest and I think we'd both be happy with our choice."
When I'm asked I usually say that Swift was the greatest English keeper to play for City and Trautmann was greatest non-English.
Incidentally, on the film this is nothing to do with Warren Clarke's script.
Yes, my Dad is 78 now and remembers watching Swift in the 1940s. But his memories are of Frank joking around with the young boys behind the goal and performing his party piece of catching the ball one-handed rather than anything more revelatory about his actual goalkeeping. You'd need to be pushing 90 now to have seen Swift pre-War, so the numbers who can assess the respective merits of the two men are obviously going to be very limited.
Funnily enough, in 1973/4, City ran a vote through the match programme for our Greatest Player of all Time. The results were published for the Stoke game on New Year's Day, and Bert won it, with Swift coming second. Details here:
https://norepartee.wordpress.com/2014/09/06/1973-74-stoke-home/
No surprise in Bert polling better than Swift, I suppose, simply by virtue of being a more recent player (IIRC, Spurs did one recently online and the legendary Dave Mackay didn't even make their Greatest XI, which outraged an older Spurs fan of my acquaintance). Even in 1973, you'd have had to be well into your forties to have seen Frank at his peak. Both of them remain up there among the all-time great goalkeepers in English football, in any case, along with people like Banks, Jennings, Shilton et al.
That poll in 1973/4 backs up what you say about opinions changing over time. Of the then-current or recent players, only Summerbee and Book made the top six, yet Bell and Lee, in terms of international caps at least, won greater recognition at the time. And given that Bell is now widely rated as the best ever, it seems particularly odd that he didn't feature - he'd been at City nearly 8 years by this point, had been a key man in a superbly successful side and was more or less at his peak, so it's not as if he hadn't proved his worth. Maybe his subsequent injury brought home to people how much we lost when he wasn't there.
I only ever saw Bell after his injury, but I'd have thought that Silva must be close to deserving the crown now, but I know others disagree. But then my grandad and others of his generation always maintained that Doherty was a class above any other outfield player we'd had. And he told me that, in the thirties, the older generation still insisted that Meredith in his first spell was better than Doherty! There's definitely a big generational factor.