I've driven a couple of IS200 sports long term. Very comfortable, though not massive inside, lots of toys - the heated seats were lovely. Nice and planted feel to it, but most definately did not like to play! It never felt happy when the tail came out, so it wasn't long before I just stopped trying. The traction control kills any sort of fun in an instant anyway (along with around 80% of the power!).
I'm only aware of cosmetic differences between SE and Sport (but I'm no expert) - The sports have a nice looking kit, lower suspension and nice looking 18" wheels (that corrode badly! Check them on any IS you're thinking of buying).
The straight six engines are exceptionally smooth, but lack real punch. Nice six speed box, but has an annoying beeping when put in reverse! The metal gear knob is also nasty to touch on cold winter mornings!
Overall a nice and comfortable ride, but not much fun. The sports do look very nice indeed though and will deffinately get you noticed in you go for the rediculously bright yellow they offered.