If I was forced to choose, I would probably go with Bloodborne just because it is such a different experience. I would watch a few videos (avoid bossfights for spoiler s) to get an idea of the pacing and style of both. If you want a traditional Souls experience, then DS3 is the way to go.
You can end up sinking a lot of time in both, but Bloodborne is ultimately probably the shorter game excluding the chalice dungeons.
Edit; Just saw that you never played DS1 or 2. Since that is the case, I actually might recommend DS3 first to get a fresh Souls experience. I highly recommend going back and playing DS1, at the very least. Games are hard, you will struggle, find the beauty in getting the shit kicked out of you until you drop a few bosses (and then some).