BOTH. they both have merits articulation wise. Size wise, Unicron is slightly bigger than Primus. Toy detailing wise, both have wonderful detailings, Unicron has them plating details while Primus has them mini cities of cybertron all about. For Unicron you have to pick whether you prefer the Ornage or Black one. Personally I prefer (have) the Black making him look more of a sinister dark lord of chaos.
Alt mode wise, Unicron dwarfs Primus again. But Primus is a very good planet sphere shape, Unicron is like an eggnog biscuit, round on top and flat below, when you have to display him ythe top round part is the front anyways, oh and you really need to get a stand for him otherwise his ring gets in the way. Primus has his own 4 spires which serves as a stand.
I say both coz even though both have something lacking, both have their own merits and would look great displayed in VS mode. Also you better hurry coz Primus is dissapearing fast since he hasnt been reissued since his release during the TF Cybertron era, Unicron has been re-released ( in his Orange Armada version) but still is hard to find since it was an exclusive to some store (not sure if its TRU) thereby limiting its presence.
Wanted AM partner Vanguard, Myclones Dirge, G1 Victory Leo, e-hobby Dark scream ( the black version), e-hobby Magnificus
Parts- AM partner Basher-side guns, G1 Actionmaster Elite Windmill's blades[I][B]