Picking a new car battery, what to look out for?

I need a new battery for a 2010 Audi A3 1.8TFSi, I went on carparts4less and entered the reg and chose the Bosch S5 027 as I have had good experiences with the silver bosch range. When I compared to my stock battery the Bosch was rated 63Ah, 610 CCA but my stock battery was rated 70Ah and 340A DIN, 570A EN/SAE.

Does the Ah matter or just the cranking power?

Dimension-wise this battery seems to fit my car and all the power figures are well above stock so might this be the better choice?
http://www.battery4cars.co.uk/manufacturer/audi/audi-a3-s3-3-2-v6-petrol-car-batteries/e44-varta-silver-dynamic-12v-77ah-car-battery.html

Cheers

Dont get one with a lower ah rating, a higher is better but dont go lower

Cheers, I ordered the battery I linked to and an Audi pro just messaged me to say that's the best you can get for my car so glad I did the extra research myself rather than rely on the sites that use your reg to find compatible parts!!