I looked at a Plat prior to buying my Lariat. Fo me, the Plat extras weren't worth the added cost. IIRC you get heated/cooled rear seats, ambient lighting, wheels, and some trim (probably missed something).

I have 3.55's in my EB and it pulls great. I don't tow, but I'm sure the 3.55 will be more than adequate for your boat. If you think you'll be moving to the upper ends of the tow range sometime soon, you might consider the 3.73. Keep in mind that the EB makes outstanding torque down low so you don't need as much rear gear to get the engine into the power band.

Edit: FYI, the 3.15 would have been just fine, but LS was only available in 3.55 and up, so that's why I opted for it.