We don't get it as cold as you guys but in my opinion, you should dress for immersion not for comfort sitting on top of the kayak. Both cold shock and hypothermia are killers and you need to protect yourself as much as possible.
If you are getting cold sitting on the kayak, your skin surface is already shutting down, if you fall in, specially if we are talking ice cold water, you will have less than a minute before your muscles start to stop working, and even if you do get back on the kayak, you still have to make it to shore and help. With legs and arms in a state of cramp, that won't be an easy task.
Here in Australia only a few weeks ago, a kayaker fell in and was washed from his kayak, I am not sure just how long he was in the water 10 - 15 minute's maybe, and when he was pulled from the water he was already in a state of hypothermia. We are talking 11 degrees Celsius not zero or minus.
I have not tried two piece dry-suits but can vouch for a 1 piece, best bit of kit I ever purchased, yes expensive but what price do you put on comfort and your life. Very important to get the relief zipper and I recommend a front entry over a rear entry suit. Latex seals are susceptible to UV light so either make sure the suit has cuffs on the arms to cover the seals or they are neoprene, same for the neck. Best zips are the YKK dry zips instead of the plastic ziplock style that you find on plastic bags.
It might take a little to get used to layering under the suit but once you do you will wonder why it took so long to get one.
Couple of other topics about dry-suits on here;http://www.hobiecat.com/forums/viewtopic.php?f=13&t=32545&p=129389&hilit=drysuit#p129389http://www.hobiecat.com/forums/viewtopic.php?f=9&t=32514&p=128416&hilit=drysuit#p128416http://www.hobiecat.com/forums/viewtopic.php?f=70&t=26642&p=110671&hilit=drysuit#p110671http://www.hobiecat.com/forums/viewtopic.php?f=71&t=23445&p=102090&hilit=drysuit#p102090