Enektric

If I were you I woud take the 15" not only because the performance is better but also because the screensize is quite ideal for both mobility and workspace (at least that's in my case).

I wouldn't worry too much about it's ability to run the game. (Especually when you have an ssd )
I have an IMac 27" i7 from 2011 and I can run gw2 smoothly Maxed out and a mbp15" core2duo from 2007 where I can run the game smoothly on low/medium.

I also found that you will gain a big fps boost when you run gw2 on osx instead of bootcamp since the graphics cards on a Mac are used as a normal processor and help the I7 out while it's doing less work vs the I7. In a game like gw2 that relies highly on processor power this makes a Huge difference.

On the price of the machine you have to keep in mind the price/quality of both.

You could buy a plastic box for 1000€ and have the same power but carrying something like that Around might affect it's lifespan,
A fully alluminum mbp might be the better option then.

My updated mesmer:
gear: T3 + necro head
weapons: bifrost, Bolt and mystic pistol

I am currently running around like this:

My mesmer in T3 Cultural gear
dyes used: Abyss dye, White dye, lemon Ice