This is actually a good question.
Usually 17:9 formats (DCI 4K and 2k) are used more for cinema projects where the screen aspect is 17:9 and not 16:9 like on TV and YouTube.
Although its funny, since they have the DCI 4K in bold, but not the DCI 2K...
Anyway - I shoot both formats, and can't really tell you what sells better.
I sell both formats on all sites.
If the clip is of good value - people will download it and doo the small resize for their needs I suppose.
Shots are rarely framed so tight they can't be fitted to another frame aspect.
BTW - the same question is asked many times regarding 24/25/29.97/30 FPS.