Look at the pros and cons of each. The 550d outweighs the 300r in pros but there is a £50 difference. Do you want noise dampening foam? What case do you think looks nicer? That £50 could be spent on a different component. Could be the difference between a 7950 and a 7970, 128gb and 256gb SSD, 8gb and 16gb RAM, a good mouse, the list goes on. Consider what you could buy with the £50. With that included, does that make your decision easier?