You'll be more comfortable in the long run on a bike that fits you properly. That said, there could be things that can make that 53cm fit you -- depends on where it's not fitting -- it depends if your height is in your legs or torso.

I know this sounds superficial, but if you don't like the looks of a bike, it could influence how you feel about riding it. I know that I love riding my bikes partly because I think they're beautiful!

With a goal of the 150, you'll need to spend lots of time on your bike -- get the one that fits you best and inspires you to go out and ride as often as you can.