I have 3 heater tops, one for each round! I have an O'Neill Hammer 2/1, an Xcel, and an Eagle top. Here are my thoughts on each...
O'Neill - I think the 2/1 refers to 2mm of torso thickness and 1mm of sleeve thickness. This is probably the warmest of my tops and is a rear half zip with a long zipper string.
Xcel - I really like this top because it has a full front zipper. I think it is designed as a SCUBA dive top. Probably not as warm as my O'Neill, but I think partially because I should have got one size smaller.
Eagle - This is my oldest top and I rarely wear it anymore because I like the other two better. This is still a decent top, but I like the other two better.