Then you have made good choices, both of those laptops are high quality and the specs you have selected should run Arma 3 at 30 fps during firefights, 40-50 when not in a firefight. You may want to look at other features they have, such as screen refresh rate and other not completely necessary things they have, but that you may want (also such as storage) to find the best system for your money.
*Dell is currently having a sale on their website on Alienware 17s*
**Edit: It looks like the Predator 17 has better specs for the price, but this doesn't say anything about build quality or customer support/warrenties**