If you are a "very-mobile user", which means for me that you will use your laptop anywhere on a daily basis, pick the one that will gives you the longest battery life and that is the lighest (probably the Dell).
But, if you are a "casual-mobile user", which means that you move your laptop from home to office only, or keep if with during some week-end, get the cheapest.
If you want to do some entertainment (gaming, video playback, etc...) get the one with the highest performance and the best GPU (probably the Acer).
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