It's a simple matter of what is easier to fix. If the only thing preventing you from getting the larger laptop is how you plan to transport it and prefer a backpack. I recommend looking at the backpacks from
http://www.wengerna.com. They make laptop specific bags for varying sizes and they tend to be very very durable.
On the other hand the smaller laptop I would assume to consume far less power (didn't look at rated battery life on either) so for your extended time away from power sources it would make more sense.
In the end I know this didn't give you an answer. It's your money and we can't spend it for you. All we can really hope to offer is points of view that will give you some insight as to what will suit you best.