I am seasonal, but I was on the CP and completed it and then went seasonal, so things may be different. I believe its rare for Disney to hire straight to Seasonal, but I guess it depends on their needs. Usually you have to have fulltime availability for something like 6 months before you can go part time or seasonal
I have a Seasonal (Red) ID which gets me into the parks for free every day. CT (Seasonal) CMs get Maingate passes (which get friends/family into the parks for free - in general 3 people at a time 6 times a year) BUT you must work a shift and get paid in the year before they send it to you.
I haven't yet worked in 2010 and I will not have my maingate for my March trip, but I'll have it if I go down again after that.
I'm pretty sure thats the only difference - I get dining, recreation and merch discounts, just like "regular" CMs and I get holiday discounts.