Congrats on the .357 Marlin. One of those is definitely on my short list.
The SP vs the S&W 28 is a bit of apples to oranges. The SP falls between the J and K frame size wise. I haven't fired a 28, but it is an N frame, so is even larger than a 686, which is very close in size to a GP100.
I have a 4" SP101 and love it. I have had a GP100 and S&W K frame, and have fired a J frame a few times. This is all very personal preference of course, these are all fantastic handguns. GP100 and 686 are a little too big for my hand. The J frame is a little on the small side for extended shooting, but a 3" model 60 is also on my short list.
The SP101 fits a Goldilocks role for me. It soaks up 357 recoil pretty well, and is quite nice for extended range time, but is a little lighter for carrying in the field (it's primary role for me), and can even be concealed IWB reasonable easily.
The SP101 comes with a great set of sights. The trigger isn't great, but can be made good fairly easily with a spring kit, shins, and some time polishing internals. It may still be a bit heavier and will have longer travel then a K frame, but I am picky when it comes to triggers, and I am pretty happy with mine after some work.
Bottom line, SP101 4" is my favorite revolver to date, personally. I would look at that, but make sure you also consider a GP100 or 686. Those are the ubiquitous, go-to 357s.