I don't think this is as simple as 1 side is better than the other just going of face value.
what do your other RB's look like? & when you say mid-range pick, we talking like pick 4 or pick 7...? I'm still leaning toward the Mendy side, but Holmes should be a good WR for at least a few more years, Sanz has the potential to be a decent slot receiver, and depending on what pick you're looking at, you could be getting a stud.
While I still like Mendy, he's had a ton of touches last year and PIT's O line looks awful.
So, on face, I'd take the Mendy side for sure, but if that draft pick could get as high as 3 or 4... then maybe I'd consider the other side of this trade