The GT like SwampAss mentioned has all those extra goodies..
I'm a gizmo geek, and I like to have those little things just cause I can. This is the first car I've ever had leather in, and I love it. I guess I haven't experienced Mercedes/Lexus/BMW leather, or any other top notch leather, but I'm diggin it.
If I were you, go take a look at both the GT and Sport trim. Make sure the GT has all the options that way you can play with all those toys in there. Granted you won't use all of them, all the time. But you'll get a chance to see first hand what it does and how much you like it compared to not having it. I didn't get the NAV system simply because I didn't like it. I ended up getting a TOM TOM XL navi later down the road for 249.00. But if you must have the Navi option too; then that's your choice.
The GT options that don't come on the Sport that I use all the time are the HID's (no kidding eh? lol) Leather, given. Bose Stereo(could use a little more umph, but thats nothing that some sound deadening material can fix). I hardly use the auto-sensing wipers; and the auto on/off headlights.(is this available on the Sport?). Other than that, everything that comes on the GT are "nice to have's. If you think its worth the extra money, and you like it, go for it.
My question is, if you already paid off your P5, why not keep it, and just get an MS3 altogether, and have the best of both worlds?
-J