John, I just got the LV Medium shoulder plane, and I really like it, REALLY like it, the adjustable mouth is a big advantage, you can make very fine cuts with it on endgrain, the shoulder part of the tenon. The shoulder plane is also more useful, as it is just under 3/4" wide, 11/16" IIRC, so it can be used to clean up dados and grooves too, like on a shelf, where you use your dado blade, but the bottom is not quite square, you can under cut it a bit, and then square up the bottom of the dado with the shoulder plane.
For cutting down the thickness of the tenon, if you already have a good block plane, you can use that for the majority of the cutting and the shoulder plane for just the little bit right next to the shoulder of the tenon.
I'm no expert, but to me the shoulder plane is more useful, I think the rabbet plane would not get used nearly as much, by I could be wrong.
I know you will be happy with the LV Medium Shoulder plane, I was concerned that I was maybe buying a tool that is too specialized to use much, but now that I have it and I see what I can do, I'm sure I'll use it a lot more than I thought I would.
The pessimist complains about the wind; the optimist expects it to change; the realist adjusts the sails.
William Arthur Ward