I was interviewed by YC, DreamIt Ventures, and Techstars and ended up in Techstars Boulder. Can't say much about DreamIt but YC is all about your idea and a quantity of companies. Techstars is really about the team and quality.
As a suggestion to get in you just need a good idea and more importantly you need to work your ass off! Before I got in I cut my second job to part time, stopped going out, almost lost my girlfriend, etc because I was putting every second of my day into my company. Make sure the other members of your company do the same because it's all about the team.
As for the optimal learning experience... during the course of the program work out of the shared space and be there all day seven days a week. You'll have the advantage of bonding with the other teams and meeting random people that stop by. Also take advantage of all the sessions and meet as many people as you can. We probably had 100 meetings with different people over the summer and I counted only 3 that weren't useful for us.
Either program is great but if you have a choice I would hands down suggest focusing your energy on Techstars.
Good Lick!