Explain the differences in the 13" and 15"

Can someone explain the difference between the 15" Retina MBP and the 13"? I'm looking at them, but I don't know what they do.

Should I be concerned that I can't up the RAM past 8 gigs and the lack of NIVIDIA and dual vs quad core?

I'm going to go for the 13" for $1899 (student discount). I'm not a fan of 15 inches but its only a hundred dollars more with the discount and I'm not losing anything. Worth it?