As I understand it, the 80 is stronger on the inside but they are the same on the outside. Since my 9 year old is right at 65 pounds and still just fits in the Radian, I am getting the 80 for my 13 month old because she is a little shorter. She may fit in it a little longer than my 9 year old and may be a little over 65 pounds when she outgrows it by height. The price on the 80 was dropped on 7/1 to $250 so it's now just $50 more for the stronger seat.
(A few years ago, I'd have been saying there is no way Kaylan would make 65 pounds. I have been very surprised by her growth in the past few years. She has always been tall and skinny. At 3 she was 30#, 4 - 34#, 5 - 40#, 6 - 46#, 7 - 52#, 8 - 57#, and 9 - 66# and 53". She wore a 2T shorts until she was 5. She is now in a size 7/8 with size 10 tops for length.)