I’ve been doing my homework on these lately; my observations are:
Ebay – full of new eBay members with 0 feedback selling what I suspect are ones with the “Yellow Light of Death”.
First gen (called FAT in the lingo) are all dying (that are for sale)
The ‘slim’ are the secondhand choice; in whatever GB you fancy.
The ‘superlim’ are lacking in same level of quality as previous (from reviews, not opinion) but however, you can get a 12GB (which although low memory, is some clever arsed HD rather than the usual solid state thing) for £129 instore at Asda, with some trendy Assassins Creed game, which will obviously come with warranty.
If this helps!