Sick mate, all I want to know is that will I be able to run counter strike 1.6 smoothly/runescape 07? These are the only 2 games I will play in the lifetime of this laptop, they are both reasonably old aswell. What will the dual core 8GB ram 2.4ghz 13" model struggle to do, like is there a list of things it just simply won't do?
Also is the 13"/15" just a matter of increase in size or does it affect colours/power/speed etc (dumb question)
The 15" offers almost twice the CPU performance of the 13" models. I think you should be looking at the 13" options though as you only plan on moving it a few years. For your needs the 2.4Ghz 256Gb SSD is best. I only brought up the 15" idea if you wanted to do more advanced things with it as well as keep it longer.
So go for the 13" 2.4Ghz 256Gb SSD option; as long as your needs don't change; it is good enough.
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15" has the Core i7 processor, whereas the 13" has the Core i5 processor.
That isn't really the main point; it is that the 15" is 4 cores hyper threading to 8 virtual; and the 13" is only 2cores hyper threading to 4 virtual cores. Benchmarks show around 2x CPU performance with the 15" vs the 13".