Is そっか just more slangy? Is そうか the same as saying ああそう? Or, is this more a spoken thing? If so, what is used when writing?
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I think that
In writing, you'd just write "そうですか". Writing (in any language) should be more polite and formal, so as to avoid misinterpretation…
Japanese speakers often use expressions aimed at themselves. Functionally it's like they're talking to themselves, which would sound strange in English speaking countries but is par for the course here. 「そうか・そっか」 is an example of this kind of expression – people make an exclamation ("Oh, OK!") to themselves when they realize something, grasp a meaning, make a logical connection, etc. In these cases デス・マス isn't necessary because it's understood that the speaker is making the statement toward him or herself, and not the other party. While the meaning is essentially the same, the use is different from 「そう〔なん〕ですか」, which is explicitly directing the statement toward the other party for further comment.
To answer your question, there isn't a difference in meaning, but I usually hear the second more often in conversation. Any time the consonant sound is doubled up with the small ッ the meaning becomes more emphatic (e.g., 「すごくおいしい really delicious」 vs 「*すっごく*おいしい REALLY delicious」.