If your a “keen mtber” as stated in your first post, you have a decent level of fitness already?
At that point, I’d go 11-25 for the tighter spacing. Road bikes are supposed to be fast, not easy. Unless you live in the lakes or wales, that will be more than low enough to deal with the odd 1 in 4 or 1 in 5. Working harder in a higher gear will do your mtbing fitness no end of good too.
I’m another “scrawny pathetic weakling” – 5’10” and 10stone and can just about cope with 53/39 and 12-25 in the Peaks. I can get up everything, but spend far more time standing than I should. A lower gear would probably be more efficient, but if it ain’t broke, don’t fix it… If I ever decide to take the roadie to some proper mountains i’ll be investing in some lower gears though!