Is butyl sealant necessary

I've had a leaky windshield for quite some time and finally decided to fix it today. I pulled the windshield and cleaned the area but I dont trust myself reinstalling the glass and being satisfied. I called a local glass shop to get a quote and ask some questions and they are telling me I need a urethane sealant rather than the butyl that I requested. I've read on here somewhere that you're supposed to use butyl on these older cars. Does it really make a differience? I want to get it done right the first time.

Also what is the best way to prep the surface so it wont leak again? Sand, primer, and paint?