The DCP website actually got a lot better since last time I looked, so check this out first:
http://cp.disneycareers.com/en/program-components/earning/role-descriptions/
These are worth reading too, although a lot of info will be repetitive usually there's something new you'll learn:
http://onegirlonemouse.tumblr.com/dcproles
http://cousinorville.com/the-ultimate-guide-to-the-disney-college-program/
There's plenty more like those too! If you find a few roles you're specifically interested in, you can usually find people with good vlogs/blogs about a lot of details, with good and bad. Some less common roles it's a little harder to find, and your best bet may be to turn back here for someone who did do that role!
When I applied, I knew I just wanted to go so I put down every role with the exception of Costuming and Character Performer. (Also, no BBB as I'm a guy.) The more roles you put, in general the easier it should be to get accepted, but don't stress out about putting no interest in ones you really don't see yourself being happy doing.
In terms of Spring vs. Fall they aren't all that different. In general it's probably a little less crowded in the Spring so that is a plus! Also, Flower and Garden Festival, and Star Wars Weekends will take place during Spring. I'm sure you've figured out Advantage is a bit longer than normal Spring! It's a blast, so having the extra couple months with Advantage is totally worth it! If you're interested in
Disneyland, their Spring program is essentially only Spring Advantage, as that runs Jan-Aug with no shorter option.
