Impossible for anyone here to answer that without knowing you, how you use a computer, and what your preferences are. What will you use it for? What would you LIKE to use it for eventually? How long do you plan on keeping it? Do you have other computers? Will you carry it around a lot? Most importantly: which one do you think would make you happy?
First things first, I'd figure out what your priorities are with regards to specs. What's most important to you? Storage, speed, weight, screen size, battery life, etc?
Buy what you need not by balancing specs. If you are contemplating a 256gb 15" why do you need 512gb in the 13". I suppose its always nice to know you have the best but it might be a waste if you don't actually need it.
Would be used mainly for uni and would most likely be using bootcamp with windows 7 aswell and some Adobe Lightroom/Photoshop/Premiere.
I have a custom desktop PC that I would use for most of my storage I suppose. I'm hoping to get the one with the longest lifespan, my current laptop is an old dell with its hinges broken
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If I get the 13" I would like to spec it out so it doesnt go out of date as soon as possible, Where as if I get the 15" I can only afford the basic configuration
Buy as much storage and RAM as you can afford.
Also, are you sure it's only 20 quid difference? Looks more like a couple of hundred to me!
What do yous think is the best value for money? There is only ~£20 difference.