IPhone 5S for £38/$63 a month, or iPhone 5C for £33/$55 a month?

I've just got an iPhone 5C, 24 months £33 a month, 500mb of data, £10 up front payment. But i've just realised that I might be able to just afford an extra £5 a month for the iPhone 5S, which would be 1gb data and £40 up front cost. So i'm tempted to swap for that, what do you think?

IPhone 5C for £33/$55 a month is better.