I'd go with 1080p. With 4K becoming increasingly popular, it doesn't make sense to produce projects at 720p anymore. Plus, I feel that taking the time to create a 1080p project gives your customers more options, instead of simply trying to save yourself a few hours by creating a 720p project.
Let it render overnight or over the weekend! It can't be that bad can it?
I don't think we're going to see much in the way of 4K motion graphics in the near future. It requires a lot more computing power - and there are currently few monitors or TVs that can display 4K, no media that can save 4K (DVD/Blu-ray style) and no TV channels that broadcast 4K. Until some or all of those things happen, I can't see it.
Generally, even footage that is shot at 4K is posted at 2K or HD.
In my experience, only feature film is regularly doing it at 4K at the moment.
Maybe give it 5 or 6 years.