They are all good choices! Don’t dwell on it.
There are actually proprietary programs in the nonfree repo, but I personally do not see the benefit of this tixati over existing libre torrent programs. They only seem to provide binaries for select platforms, so it would be hard to maintain.
Also I think you may have misunderstood the relation between AUR and xbps-src – a better comparison is to abs or maybe ports. The AUR is just a collection of PKGBUILD’s for abs. xbps-src does not have any extra packages compared to the void repos. Though you can add your own or others’ templates much like you would use a template from the AUR, and anyone can submit a template to void-packages somewhat like they would upload a PKGBUILD to AUR. If it’s of good enough quality (it probably is), anyone can use the package directly from xbps-install, without jumping through the hoops of the AUR.