Wow, pretty strong statements here. I like working with the putty tape better. I picked up some butyl tape by mistake once and found it difficult to work with. Went back to the trailer supply store and got good information about the benefits and advantages of both types. For my application, on both my trailer and older tent trailer, the putty was actually better.
I do like the advice to "use what was there" before. I can't remember what the guy told me but it had to do with possible staining and black streaks on fiberglass siding. Again, talk to someone in the know.