The exhaust won't affect the MOT but decat will, so if you cant be pestered with changing it in out go for sportcat. They are quite expensive about £450 brand new. There is only about a 5-6 bhp increase in power with an uprated exhaust so standard is fine. The sportcat gives the car 15 bhp more and gives a better burble. I have the GGR sportcat on mine.

If you do want to upgrade the exhaust the most popular are Milltek and Mongoose, there is arguements for and against both. I went for the Milltek but given my money back I would go for a Primary Designs item, but again they are more expensive at about £550. Milltek and Mongoose are about £440

Hope this helps you mate.