I started with a MASHO and when I got my second packer, I went longer, and I liked that decision. Since the MASHO, I've had a 5" Doc Johnson Pack It (it's supposed to be 4.5" but that bs it's a little over 5") and the RM basic packer. I personally would pick the 4.75" for easier access through fly- I wouldn't be sure if the 3.75" would clear the fly for me. However, I think both options are perfectly feasible and I know I prefer a slightly larger packer, which is why I would go for the longer of the two. The shorter is still a perfectly reasonable size and I think they're both very viable for your situation.