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@Ndeipi
You'll probably be fine on the 12-25 until you hit something really big, then you'll briefly wish you had more gears. I did a rainy 1km stretch of 10-15% in the Brecons using 39/25 and I wished I'd had lower gears. But then I never any problems before or after this HTFU moment. For fitness comparisons sake, I was cycling probably 120-160 miles per week and usually did my 10 mile commute in around 30 minutes.
If you ever carry more than 8kg in a backpack or panniers then definitely opt for the 12-27, otherwise just think about what gear you usually use in serious hills and search for it on an online gear chart, then buy the cassette that allows you to reach that in your new 39 ring.