Using FP+ for MSEP may not be the best use of your first three FP+, especially if you arrive at MK early. Here's why: For FoF, using FP+ made sense for me. I arrived at MK early afternoon that day, so I could easily fit it in as one of my first three, the first being 7DMT just before it. It is a great viewing spot. You can look right down the entrance from Liberty Square and see the parade turn onto the hub.
As for MSEP, if you arrive at MK early and MSEP if one of your first three FP+, you cannot get any additional FP until that third one is used that evening. You're better off getting three other FP+ that you can use "back to back". Then, if you're lucky, see if you can get a fourth or higher FP+ for MSEP from the kiosk. OTOH, if your schedule has you arriving at MK in the late afternoon, early evening AS YOUR FIRST PARK for the day, then getting a FP+ as one of your first three for MSEP makes better sense.