The Purgatory looks like a rip-off of the Maxxis advantage. Ii think I’d also be sticking it out back with an XR4 on the front. I’m not sure you’d save that much weight over your current tyres doing that, but it would roll faster anyway, which may be what you really want rather than ‘lightening’?
I ran a XR4 up front for a bit and it was decent enough, but then we don’t really get proper mud round here, so mostly I just stick a Rock Razor on the back and something bitier up front, currently a DHR2. About to try some Wolfpack tyres on an alternative lighter wheel-set, but still waiting for a freehub body to turn up. They, the Wolfpack tyres, look promising in an old skool Schwalbe race tyres sort of way.