Seeing as how the Two Pokemon are COMPLETELY different, It totally depends on the team they're with.
Gengar makes a great special sweeper, and has high speed with a large variety of powerful special moves. It has low Defenses However. Gengar is very Powerful since it can learn ALOT of surprisingly strong moves that still cover basically all of it's weaknesses, and the speed is great. But of course, it isn't the longest lasting Pokemon, and has no significant defense. Levitate also helps it last much longer against Physical Sweepers with Earthquake.
Steelix is much more Defense focused, yet still has strong Attack, but with very low speed. It has low Special Defense however.
If Steelix is trained with EVs in special Defense or HP, or holds Resistance berries, it becomes very hard to kill as it has VERY high survivability against any Pokemon without a really strong Water or Fire type attack. Steelix is an all around good Pokemon, The few weaknesses it has are Lethal, but If you're in control of a battle and have eliminated most of it's threats, Its extremely hard to beat.. It is similar to Aggron in many ways, But Aggron has Higher Attack, and Steelix has Higher Defense.