If they are both the same Ampere Hour and Cold Start capacity, then usually all you are paying for at the dearer price will be extended warranty? Keep in mind you will have to send it back if faulty, when purchased over the net.
A few online sellers publish the weight of their batteries, so as generally the heavier the battery the better go for the one that gives you most lbs for the £.
Try another motor factor. If it's for your A4, you should be able to get a decent brand battery for £60. Audi batteries are a common format so they are cheap enough.