I think you meant 1080 by 3840 if it's over/under, since home 4K is 2160 x 3840; side-by-side would yield resolution of 2160 by 1920. To crunch the numbers in terms of megapixels, both would be 4.15 MP.It might also be possible to release an over/under 3D HFR version on a UHD disc. Assuming most if not all 4k 3Dtvs can combine over/under images into 3D.
Passive 3d already halves vertical resolution, so by going with over/under there wouldn't be any additional resolution loss. A resolution of 3840 x 1920 would seem an acceptable compromise. Although it would be a relatively small potential audience capable of taking advantage of this.