In short, yes, the 3mm suit is plenty warm for 70 degrees. However, without knowing how cold you get, I cannot truly answer that question. I stay cool, but not old, using my 3 mm in 50 to 45 degrees.
I would stick with the average sizing. Akona stay very true to its sizing chart. If you will fit in a Medium, according to Akona, get the medium. There is a big gap in sizing, so you risk having the suit too baggy otherwise, which will end up defeating the thermal properties. A properly fitting suit will be snug, as it should.
Hope that this advice helps. I know that I absolutely love my Quantum Stretch.
Igal V Thanks for the info, Jonathan