Wait for 4k to become mainstream.
Buy a 1080p now as it will be ages for 4k media, broadcasting, etc.
It all depends on your budget, if you have a TV now or not, what you'll be using it for, etc. If I didn't have a TV now, I would grab a 1080p, as it's significantly cheaper, and I can put the extra money into a sound system or gaming consoles or any extra add-ons. But I currently have an amazing 70" 1080p Samsung TV, along with current and Next-Gen gaming consoles, as well as a great 7.1 sound system, so for me, personally, I would upgrade to a 4K when the price of the 80" ones drop to around $5,000. But speaking on behalf of society, and not my personal idea, I would say buy a 1080p, as there isn't much of anything to be displayed in 4k yet other than very few PC games, and the price just isn't worth it right now.