iPod Classic - Differences between 6th, 7th and 8th generation

I am considering buying an iPod Classic 160gb. Various retailers are offering this as 6th, 7th or 8th generation. Whilst I realise that 8th generation is the most recent, does anyone know what the differences are between 6th, 7th and 8th? (This does appear to make a difference in the retail price). I don't mind buying a 7th generation if it just means it was manufactured a bit earlier (and allows me to save money) but I may not want it if generation 8 dealt with known bugs, for example, in generation 7). Any help appreciated.