Hi Louise,
As your smaller, a lighter bike will probably make more difference to you. (Being smaller, you will carry less weight, so the bike will make up a bigger part of the bike + rider package).
My partner too is small, just a little taller than you at 5'3. She struggles with reach on gents frames, so we have always looked at womens designs for her.
Are you buying this bike yourself, or through a cycle to work scheme?
Could you stretch to something like a Specialized Vita? You can get the basic 2008 model for £349 and the sport for £399, maybe even less if you look around. It would imagine it to be a fair bit lighter than either of the bikes above you have suggested. The sport certainly will be, and comfier too by virtue of it's carbon bits.
Evans have the small in stock. I don't know whether it is small enough for you. Dimensions should be on the Specialized website.