There are some hints in YouGov’s poll that memories of the Cold War may be a factor. Americans aged 18-29 were the age group most likely to credit the USSR, though they still prefer the US by 36% to 16%. However Americans 65 and up, who would have mostly come of age during the heart of the US-Soviet rivalry, pick the US over the Soviet Union by 66% to 9%, a much wider margin.
https://yougov.co.uk/politics/articles/12228-Britain-America-disagree-who-did-more-beat-nazis