Hi,
Welcome to the forum. The difference will be that the T5 units are brighter, perhaps by up to 50% depending on reflectors. Brighter light automatically means higher probability of algae, faster growth rate, higher requirement to keep on top of CO2 and nutrients. For any beginner I would definitely suggest the T8 tubes, but other than the scientific implications, lighting is an emotional issue. Brighter tanks just look nicer when things are going well. The 50 quid will buy you more responsibility and headaches, but will also buy you an escape from the dreary winter days...
Cheers,