"Bitterer" isn't a word.
"More bitter" is better(est) grammar.
Actually, Dalriata, bitterer and bitterest are both words. Dictionaries that list comparative and superlative forms list -er and -est for bitter; dictionaries that don't list -er and -est forms for nouns that have them are silent on bitter, implying that -er and -est work. http://dictionary.com/search?q=bitte
I found this survey because I read a phase using "bitterest" and I googled the word to see if the useage was correct.
To me bitterer sounds repetitive, so I prefer "more bitter and the most bitter"