monitor calibration tool - what is difference in newer calibration tools? Spider V2 v V4

Hi,
I just picked up a new mac cinema display 23"...love it, but I need to calibrate again. I have been renting the spyder tool from a local rental place over the years for like $15 per day. 
I figured I should just buy one to keep calibration updated. I was looking for used tools and saw that the older Spyder tools were only like $15-$50 used depending on the version. 
I have been using one over the years and I'm guessing it was one of the OG or V2 models of the calibration and that seemed to work just fine. 
I'm prob just going to get one of those, but just wondered if there was something that I would be missing out on by not getting a newer version. 
I do a lot of photo editing (stock stuff for Getty)...and send prints out to my lab...no layout or mag prints or home printing. Still I would like a monitor that is as close as possible. What would I benefit by getting a newer version?
Paying $30 for my own calibration tool seems just fine to me...and I'm not set on Spyder either....I think I've used eye1 in the past as well.
Thanks for the help!