If it first pack, first pick Ob is the right choice. In sealed it comes down to the rest of the pool. But just comparing them side by side, Ob is the stronger. As said he can win games without attacking and is resistant to Hideous end. Ive actually won several games with him without attacking. Add the sick interactions with land fetching or just the fetch lands and we have a winner.

On the off chance that I would get both of them in a pack(one has to be foil for this to work), I would probably pick Ob Nixilis for many of the same reasons listed here. Also Mr. Ob can grow with cards like Harrow, Kor Skyfisher, Narrow Escape, and all the rest of the land fetch/rescue cards. On the other hand, unless you have auras or equipment, Terra Stomper will most likely stay at a 8/8. The only thing it has over Ob Nixilis is the fact that it tramples. I would probably play both in a sealed deck if I had enough of both colors and a decent was to get both out.

I think i would've take Ob nixilis. the current format of zendikar is very very fast format to play, and i think ob nixilis is more suitable and strong. as it was said before, he can finish games this way or another just with his ability.