Which one is the easier to learn? I imagine the stand up backwards.
There aren’t too many hardcore Freestylers on here, so not sure if you’ll get any experienced responses. I have minimal experience, from many years ago. My gut is that sitting down is going to be (a lot) easier. But neither is going to be even a little bit easy. 
I’d say Arthur Richards would be a good one to ask. His backwards coasting on Unicycling is not a Crime is pretty sweet.
I’ve done some tiny forwards stand up coasts inspired by Arthur but I don’t make it very far compared to him. I imagine stand up backwards coasting would be far harder than sitting down since you can’t smoothly ride into it- or maybe you can if you are good at jumping up.
Coasting is such a nice feeling! Keep at it.
I prefer to sit. But you NEVER should apply statistics or opinions of others on yourself, before trying yourself.
Here’s Daiki, sitting ft in and 1 ext:
http://www.unicyclist.org/cont/play.cfm?pi=f320240bwcoast
http://www.unicyclist.org/cont/play.cfm?pi=f320240coast1ftbw