I'd go with the newer one, all other things being equal, but I'd make a much lower offer, just to test the seller.
Try offering a couple grand less. You do have to have that spa moved, and they weigh somewhere around #900 empty.
I also woulldn't get that worried about not having a warranty. Look at it this way- you'll be getting a quality spa for thousands less than new.
You could spend $10K on a new one easy. For that $6 to $7K "savings" you could pay for a couple service calls and still be way ahead.