Spyder - express vs. pro vs. elite (hardware only)

[DanPower]

Hi guys, I'm looking at getting a Spyder... I've looked at the X-Rite stuff but it's just way over what I want to spend. I'd like to spend under $100 if possible (am looking at used items), $200 is a stretch. Brand new B&H have the Spyder4Express for $114, Spyder4Pro for $169, and a Spyder4Elite for $249 (but that's starting to get outside my budget).

I was hoping someone with more knowledge on this could help.... what are the hardwarde differences between the three models? I intend to use dispCAL/ArgyllCMS so the software differences are of no concern to me. I'm just interested in the differences between the actual hardware sensor units. Are they significantly different? The only obvious difference is the lack of the ambient lighting sensor on the Express.

Also was there a significant change in hardware between the 3 and 4 models? Spyder 3's are going fairly cheap on ebay and as the software isn't important then if the hardware is the same I might as well get a 3.

Thanks!