To put it clearly, 70's Magus would win. Thanos and Warlock (and Gamora too for that matter) together could not defeat him. He was Adam Warlock's insanely evil hyper-evolved self from an alternate future. And yes, that was the Soul Gem on his forehead. He was only stopped by Warlock committing suicide to pre-empt his transformation into the Magus.
90's Magus was nowhere near that powerful. He was an evil fragment of 90's Warlock without either the Soul Gem or gemless Warlock's full cosmic powers. His big feats were all accomplished via powerups and allies. He would loose.