Well in Jadzia's case, we're dealing with basically Terry Farrell being unable to act. I had said from the very beginning that she seeed like some model strutting the runway, cold and distant. I found out a few years later that she was actually a former Ford model. So her being on the somewhat frigid side might have had something to do with the fact that she just didn't know how to express herself. It is possible, however, having a bath 500 year old Symbiont, she was intended to be a bit indiscrutable, and thus distant. Who knows.
With Seven of Nine, however, we're dealing with a former cyborg - one who was rendered that way since early childhood. So emotions were irrelevant to her for most of her life. That was the deal with the character - her becoming human again. So being emotionless was written into the character as her journey was her trying to reconnect with them.
As being a Vulcan, T'Pol, not feeling emotion, let alone expressing it, was central to Vulcan culture.
On the other hand, like Seven of Nine, T'Pol had no problem expressing her body. In fact, she was literally featured in the most steamy scene in the whole franchise. Ever.