In reply to elliot.baker:
Depends how good you are a looking after boots.
If you are diligent at cleaning and waxing boots then leather is great. If your idea of boot care is bashing the mud off the soles against a wall before bagging them until you want them a week or two later then leather is not the answer.
One thing to remember is that the synthetic boots rely on the waterproof liner (gore/event...), and the guarantee on gore-tex liners in boots is for the reasonable lifetime of a product which is totally subjective. If you take a battered/poorly looked after 13 month old pair of boots back to the shop because they are leaking you probably won't get very far. If you take a well looked after 13 month old pair of boots back because they are leaking you will likely get them assessed and replaced.