Steelix TDO as a Defender: Iron Tail vs Dragon Tail

GamePress has both IT/C and DT/C listed as the best movesets for Steelix as a defender. Though, I'd say DT/C would be a better option for Steelix for neutral coverage against Pokemon that can counter him and resist his Steel-type moves. You should also consider which quick move you would want to give your Steelix depending on what weather you normally get. If it's usually windy around you, DT would probably suffice. If it's snowy, then IT.

SIDENOTE: Rayquaza is most likely around the corner, so that means Steelix and Aggron will most likely be the best counters to the legendary dragon. Steelix will probably be able to put in maximum damage output against Rayquaza with DT/C, so also consider this, as well.

In my opinion, I'd leave your Steelix the way it is. Dragon Tail will give Steelix better neutral damage coverage (and a nasty surprise for the player who tries to sweep the whole gym with Dragonite), but at the cost of STAB. And given that Steelix isn't known for hitting all that hard anyways, I'd say that TMing it would be a mild upgrade, at best. And since you're not swimming in TMs, I'd advise against the moveset change.

To me, Steelix has two jobs when placed in a gym: be obnoxious/waste the attackers time, and decay at a slower rate then the big fat Blisseys, Snorlaxes, and Slakings. Regardless of it's quick move, it should achieve that goal just fine.

P.S. if you do decide to TM your Steelix, it might not bad a bad idea to aim for Dragon Tail/Crunch, as that's probably the most obnoxious moveset to fight against.

Use a pidgey and don't waste tms on gym defenders

Just evolve a new one.

Leave it as it is, it can be used against Gardevoir, when prestiging comes back.

Machamp is the most typical Steelix counter and does not resist steel moves, so there is that. Then again if someone brings a fire or water type to the fight, DT would be much better.
I consider FTM to be pretty cheap since they drop so much more often from raids, but it comes down to how many you have. It's a decent upgrade but not as great as FTMing Blissey, Snorlax or Chancey.

I always use 100% Steelix as a defender with DT/HS and leaving it the way it is :) 1. it has slightly higher EPS. 2. stupid people always using dragonite get hit harder :D 3. clever people using strong Fire DPS - have resistance to steel

Sure wynaut

What's a "significant marginal" impact?

Any choice you make is going to have its good and bad match-ups. As long as you don't have Earthquake as a Charged move, you're probably good.

I chose Crunch because it goes off more frequently and more quickly and attackers might get hit by a couple in their haste to power through an annoyingly bulky Pokemon.