Pro C has the same prebiotics as AviPro, plus the probiotics. ProC professional is a more concentrated version of ProC, but essentially the same ingredients.
I've found that if I add ProC in their water bottles, it affects the water quality - I guess the prebiotic stimulates the proliferation of other bacteria in the bottle. This is why I prefer to give it to them in a syringe, and I like the more concentrated version, so I can get away with giving them less.
Btw, I think Oxbow's joint support is really good. I have a young boar with arthritis, and we're managing the condition solely with Oxbow Joint support + Glucosamine; we haven't started him on metacam yet, as he's still doing fine without it. Before starting Oxbow Joint support, I think he was uncomfortable and he would occasionally bite his knees, I assume because they hurt, but I haven't seen him do that at all in over a week now, and he's much more active. He even popcorns a little

I've ordered more of Oxbow's natural support tablets, I used to think they're just expensive treats, but now I'm convinced.