Nexus 5 is manufactured by LG, but the official... producer, distributor is Google. So, as in the case of previous Nexus phones, the support will be available for longer time. M8 is totally new phone with fantastic look and camera. Unfortunately, none of those phone are water- and dustproof.
You can also take M8, then sell it and buy Nexus on e-Bay. M8 is still a bit expensive, so you will gain some money :P
Current device: Samsung Omnia i900, Windows Mobile 6.5 21889 PL
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I am from Poland, so my English is not perfect.