Long time no post to the Pentaxforum, because I switched into Nikon a couple of years ago...
Anyway, I've got both D500 and D750 and I've been very happy with them both. I think they're an excellent combination of FF & DX properties, as well as AF capabilities. For football I usually have the D750 with the Nikkor 200-500mm f5.6 and D500 with Nikkor 70-200 f2.8 VRII. The D500 is an absolute beast for sports and action, but I also use it for stuff like this, together with the 200-500mm glass:
Instagram
D500 is the more solid one of these two, with no pop-up flash etc. It just feels a solid block of well built machinery. However, I've never felt that the D750 would be a toy, either. I've shot a couple of matches in absolutely pouring rain with it, as well as in freezing temperatures, and haven't had any issues. The AF is quite good, but the keeper rate with the D500 is higher due to both the better AF as well as the higher frame rate.
All in all, you can get excellent results with both cameras.