As a native english speaker, saying "sourer" is strange for me. Either option you've presented would work, though. Personally, I would say "more sour" in this case. Saying "sweeter" is just fine to me and so is saying "more sweet", although saying "sweeter" probably sounds more fluent. Example sentences: This candy is more sour than that candy. This candy is sweeter than that candy. The lemonade is more sour in this country. The lemonade is sweeter in this country.

'sour' is a diphthong. as such it is functionally a double syllable word. so comparing it to 'sweet' misses the point. contrast it, rather, to 'loud' or 'taut'; both of which are monothongs and 'louder' and 'tauter' are correct. 'sourer' may be defensible but it is difficult to pronounce. even in written works it can cause you to pause instead of reading smoothly along.

Hello Vale. You should say 'sourer' and 'sweeter'. Always use this form to make comparatives from adjectives that have only one syllable (unless they are irregular like 'good' and 'better'). (You will find instances however where people don't follow this rule. As a native though my recommendation is to follow it, as the sentence will sound more natural and well-formed).

"More sour" is acceptable in spoken English because it sounds more fluent. However, I would use "sourer" in written English in compliance with the following rules: 1 syllable = add "er" 2 or more syllables = add "more" 2 syllables but ending in y = drop "y" and add "ier"