In informal writing you can say: "What'd you say" or "what d'you say?" When do you choose the former or the latter? Which one is more common?
They might be pronounced differently.
"What'd you say" might be pronounced with four syllables, as something like [wəɾədʒjəseɪ]. Compare the pronunciation of "isn't" which often has two syllables as something like [ɪzənt], [ɪzənʔ], [ɪzə̃ʔ] etc.
"what d'you say?" would I assume be pronounced with three syllables, as something like [wədʒəseɪ], [wətdʒəseɪ] or [wətʃəseɪ]
However, "What'd you say" can also represent a more compressed, three-syllable pronunciation (as in a line from the Imogen Heap song "Hide and Seek" (around 2:50 in this music video) which Google Play Music's lyrics section writes as "Mm, what'd you say?"), so they don't necessarily represent distinct pronunciations. (Different lyrics sources do give different spellings for this line.)
"What'd you say" has to be interpreted as something like "What did you say" or "What would you say". It could not be interpreted as "What do you say".
"What d'you say" is likely to be interpreted as a contraction for "What do you say", although I suppose it's possible that some people might use it to write the contracted pronunciation of "What did you say". The spelling "d'you" is often used to represent the compressed pronunciation used in "How d'you do," which represents "How do you do?" The only context I can think of where "d'you" seems likely to be used to represent a contracted pronunciation of "Did you" is at the start of a sentence, as in "D'you eat that?" to represent "Did you eat that?" (related Movies SE answer that uses this spelling). When there is another word before it in the same sentence, I would usually expect the contraction of "did" to be written as 'd and not d' (as in "Why'd you eat that?", not "Why d'you eat that?").
Side note: I would agree with the comment by tchrist saying that these are not exactly "informal writing", depending on what you mean by that. (It seems to me that the word "informal" can be used to mean either "not noticeably formal", or it can be used to mean "noticeably less formal than ordinary speech".) I would only expect to see written contractions like these in written dialogue, specifically, or in highly informal situations where people may try to indicate the sound of spoken language like online chat or text messages.
In a regular online forum post, or, for example, a post on this site, I wouldn't expect to see "what did" abbreviated as "what'd". It would seem a bit inappropriate to me in the same way that the use of spellings like "wanna" and "gonna" seems somewhat inappropriate to me in these contexts. (There are some senior members of this site who sometimes use spellings like this, but with them I can assume that they are intentionally trying to give their writing a "folksy" or informal/colloquial feel, while when a learner uses the spelling "wanna" I never know if they want to come across as unusually informal, or if they just don't understand that native English speakers use this pronunciation much more often than they use this spelling.)