Advice on 2nd gen purchase

difference between 2nd gen and 3rd gen batteries?

what mileage/year should i get in either? I hear 2nd gen batteries have average life span of 8 years, and often last longer. And have warranty of 100,000. It is hard to find a used prius under 100,000 in my price range. I'm thinking a 2009 for about 6800 max. Or... If the 3rd gen is significantly better for other reasons, maybe making a little bit more of an investment... I need to see that its worth it though. Someone told me the 3rd gens often have a head gasket problem. That's a bloody mess to fix and its very expensive as well.

is the replacement cost of the battery worth it just for the added features of 2010 3rd gen models?
a off-brand company i found in Houston installs brand new 3rd gen for 2,400
and sells refurbished 2nd gen for 900

I don't know what to expect from the batteries, or how to know when theyre going to crap out... But do you still save money buying the replacement battery for the 3rd gen batteries?

I did a calculation for the 2nd gen with current gas prices where I'm at and compared it to a toyota corolla and got this:

47 mpg/3.30 per gallon of gasoline
to go 50,000 costs roughly 3,500
- replacing battery with refurbished 1000$ battery ends up still saving about 1,200 in gas. (and thats expecting the battery in a used car I buy still gets 50,000 miles)

in a corolla getting on average 29 mpg
29 mpg/3.30 per gallon of gasoline
to go 50,000 costs roughly 5,700