Thanks for the replies, my current rims aren't all that wide, I don't think, so think I might go for the 355's. I'm not sure if I'm a good enough rider to feel the difference between the SP and standard hubs, the main reason I was looking at the SP hubs was because I preferred the look of them!
Getting the SP wheels, including all the rim tape, valves and sealant from Winstanleys is about £25 more than Wheelpro, and Wheelpro wheels will come with the the rim strip and valves fitted, plus the sealant, axle swap, and a spoke tension check as well. Still wondering if this is a better idea than the Winstanleys deal. Decisions, decisions!!