jackets and trousers def... I go all the time with loads of kids and they all wear jackets and trousers, much easier - biggest problem for them when we go in Jan is cold fingers, many tears shed from cold fingers so ensure some nice warm gloves.....
If my son was anything to go by when he was seven then a straight jacket. To answer your question then I would say jacket and trousers, more flexible (IMO) as you can still wear the jacket at others times when outside.
Well i would have to disagree, my wee boy is 6 and i put him in an all in one suit. This is because in previous years i had the jacket and trousers but as he was only learning and falling quite a lot the snow would sometimes go up under the jacket and soak his top underneath. Thats why i think the all in one is better because if he is falling about then he stays nice and dry.