Henry, as a new home owner and not a big fan of caulking (I make a mess)I needed to recaulk our fiberglass shower walls, where it meets the tub.
I used the wider one and did not remove the old caulk. The old caulk was sealing and we had no leaks, it was just moldy. I didn't want to mess with taking it off, putting on the Caulk Strip and finding our that it didn't work and end up with a leak.
So far no problems and it went on very easily. I would recommend bending it to make the L shape before taking off the backing.