In general 4k/30 is better. In fact some cameras such as the GoPro Hero5 series have a significantly poorer video image (esp. in low light) at 1080p/60 than at 4k/30 -- unrelated to the resolution. Many higher end 4k cameras which can also shoot at 1080p don't deliver a good image at 1080p -- much worse than the prior generation which could only shoot at 1080p. In those cases, whether you want/need 4k or not, it's best to shoot at 4k then transcode to 1080p. This varies by camera so there's no single answer but I'd be cautious about shooting 1080p on a 4k camera unless you have done extensive QC testing to verify the image.