Are you going to be adding a discreet GPU to the build or just using the integrated GPU?
The i5 3550P has no integrated GPU, so it will require a discreet GPU. The i3 has Intel HD graphics on board, and it's not very good for high def gaming.
The best setup would be the i5 3550P with a discreet GPU.
Definitely a discrete GPU I have an old HD5770 (not the best I know) kicking around, but the addition of a windows upgrade and potentially a new GPU if the HD5770 doesn't work brings the price up by around ~£150 so thats why I'm apprehensive about the i5 due to it being £45 more expensive than the i3