Alice Miller in one of her books (can't remember which one but possibly the famous 'Drama of the Gifted Child') talks about how she specifically prefers to use patient because that conveys the real meaning of therapy Miller style whereas client implies something more business-y / less serious etc (not sure if I'm paraphrasing her right since it's been a while). Then again, I've only read her in translation -- so, who knows?!

ETA: Given my adoration of Miller, I'd go with patient -- current T says client. Potential new Ts -- haven't asked.