what is the difference between airport extreme generations?

It's a 4th Generation model that was originally introduced in Oct 2009 and discontinued in June 2011.

The current 5th Generation model uses new chipsets for the wireless radios and improved, additional antennas for better range.

Whether or not most folks would notice a real difference in day to day use between Gen 4 and Gen 5 is open to question.

If you can pick up the MC340 for $80-90 or less, it might be worth a look.

But, you can get this same model as a refurbished unit...that will look brand new.....from Apple for $129 with a new warranty.

And, if you want the current Generation 5 model, you can pick that up for $149.

The $129 refurb.....which will look brand new.....would be interesting, if you want a 4th Gen model.

Or for $20 more...at $149.....you get newest 5th Gen with new warranty.....a good deal as well since it will look brand new....and you have better antennas on that model.

If you ever have a problem....I think that you will get better service from Apple when they see that you purchased directly from them.

But, your call. Good luck.