I'd skip the rear shocks and get some parts store truck shocks. Works out to about the same thing but cheaper. Best price I found on the OME rear shocks was $60 each or somewhere like that...I can get nice rear shocks that fit great and are monotubes instead of twin tubes from the local NAPA for less than $25 each.
Medium is the standard suspension and HD is if you plan on running around with heavy bumpers, winches, lots of gear/stuff in the back, and/or tow a trailer. I was thinking the mediums up front and the HD springs in the rear for when we tow with our GV.
You get less lift I believe because of the weight difference between a GV and a Sidekick. There's a much bigger engine and drivetrain on the GV..not to mention the body got a bit "fatter."
Haven't got the kit myself but I've never heard a bad review of the ride of the OME stuff. Always positive, the Aussies really do their homework.