Use the 1/2 inch if you can, it limits gaps and leaks. Also, leave the paper backing on the window side and test fit the window, look for gaps and use some more sealer in that area(s). I roll thin pieces between my hand to make thin ropes to fill in the gaps. Once you are happy with the fit around the window, remove the paper backing and put the window in for good, it helps to have a helper with this step, or you can use wood blocks, I use paint sticks. Try NAPA, they have the sealer.
Also, use the search function on the site and look for threads on the subject, they are on here and they helped me alot when I did mine.