(Stealing your comparison pics, Peedi)
We've got two solid Sabretooth entries here. Personally, I'd give the edge to XM. I feel the base works perfectly with other SS villains (Mystique, Magneto, Juggs). Also XM's gives me the impression that Victor Creed is more than just brute power. He looks more agile and is closer to the "contract killer" vibe I grew up reading.
SS is definitely more "beast mode" out in the wild having just killed and eaten a deer. And I like that version from time to time. However, Martin crosses the line for me. I can't speak to anatomy but the proportions look more like a Hulk statue. I preferred his first two takes on the character's physique (the 1st PF and the diorama).
Of course the XM will be much more $$$ than SS. So that does play a role in people's buying habits. I won't be buying either because I can't commit to both DC and Marvel PF series anymore.