It depends on what you'd want to use each one for. I'm a major fan of Evolite Onix, seeing as it's great for stalling and setting up in a Sandstorm. It can us Stealth Rock, then Toxic, and Dragon Tail the opposition out of there for Stealth Rock abuse and Toxic spreading. Onix is a great UU/OU force at the beginning of a battle.
Steelix is a bit less versatile in defenses, seeing as Special-based attack will KO it. However, Steelix has a much more solid typing, more HP for more physical bulk, and better offensive stats. However, it lacks the stalling potential that the new and improved Onix has.
Really, it's a matter of whether you want this slot used for stalling and passive damage (Onix), or straight out physical tanking (Steelix). Take your pick, Bud!