Hmm, the differences to me are:
Prep + prime has really fine sparkley particles in it, and produces a bit of a dewy glow, but definitley not over the top. The finish is silky and soft (like any other silicone based primer). I absolutley love this stuff, its the only silicone primer that doesnt break me out, doesnt make me look greasy, and definitley improves the look of my foundation.
strobe cream is pearly, not sparkly. it gives even more of a dewy glow than prep + prime, but was a bit too much for me. It kind of just made me look like a grease ball. I found the finish to be a bit sticky, but not terrible. I think it would probably be good for dryer skins.
moisturegleam I *think* is meant for use on the body, and has more of a golden (but still pearly) look to it. I dont own this, but I tested it at the store and it didnt feel like anything I would want to put on my face; I think it has a heavier texture than strobe cream.