I would do both...write an email and discuss it at your next session, assuming emails are OK with your therapist. Sometimes I've found therapist responses to emails quite unhelpful...either they misunderstand the point of the email, or they are in a hurry and so write something short and useless, or sometimes they take forever answering and I worry about what they are thinking. So I've started just saying something like "I really feel like I can explain this better in writing, but I don't expect a response. I'd like to talk about it next session.

But your therapist might be much better at writing responses than mine. Either way though I think writing an email is a good first step to better communication.