From a limited experience with either of their writings, I don't think it's Szabo. I've read a lot of Satoshi's emails and Satoshi's writing style seems at the same time less academically formal but also more technical. Nick Szabo's writing, at least on his blog, seems less technical but more academically formal. Nick Szabo also seems to heavily discuss history and historical concepts. I do not see a lot of technical content on his blog. Satoshi seems to have a greater command of - and focus on - computers. This doesn't rule out the possibility the Szabo is some kind of Newton-like polymath who has command of a vast quantity of knowledge, but it seems more likely that Satoshi is more technical-minded than broad-based in his knowledge, despite being very smart. They might use a lot of the same phrases and idioms but this could just be a function of the fact that Satoshi was heavily influenced by Szabo. Regardless if they are writing about the same topic they are likely to use a LOT of the same words and phrases, and it might be more useful to filter out any words or phrases that are specific to the topic at hand.