Sorry so late in repsonding. Butyl will always stay pliable. Yes it is messier to clean up but with it staying pliable, it fills all the gaps. Urathane starts as pliable but then hardens. If it doesn't find its way into a gap, you are no better off. It is very hard to clean up because it hardens and is a bear to remove. Just my opinion as I am no expert. Just giving you past experiences with both.
