I went with writing, followed afterwards by a call. I think that this was definitely the best way to go for me: it allowed my mom some time to process the information on her own before the phone call (during which she still cried pretty much non-stop...it was hard).
Since I'm an academic writer, my writing style is naturally "impersonal," so I shared the same worry that you have. But I tried to make it as personal as possible, and I think I succeeded. Just focus on having a written dialogue (even though it's all you speaking), rather than just "telling" her a bunch of things. Try to anticipate the sorts of questions she'll have at various points, and then insert your answer. Then get on to your next point, or continuing the narrative.