I do agree with the benefit of a warranty on a new bike, I cracked the frame on a focus cayo and got it replaced under warranty.
Having said that you'd imagine that a good visual inspection plus a ride to check would show up any cracks present in a used bike? I'm no expert so that statement may be incorrect but I was basing it on the fact that if the seller was selling due to a crack then that's how they would know the crack existed?
Perhaps it's unlikely then to buy a used bike with a cack, perhaps it's safe enough to assume that a good used bike wouldn't have any cracks. Ok it may develop one and you may have little recourse with the manufacturer.......but is it likely?? Probably not
I know little about bikes but for me I would go giant, simply because I have a defy composite 1 which compared to the foucus I had is excellent, I also like what I read about giant and their approach to development and design. I've just gone through a similar selection process but ultimately after reading and reading I went for a giant again