Imo it completly is dependent with which wether starter you have and what role you want.
Ok so Gliscor is great with Tyranitar it covers each overs weaknesses and slop a Ferrothorn in there and you have a brilliant core.
Though Skarmory can work equally well with Tyranitar but remember it can only pull off one set and is also more vunrable to Terrakion.
Though Gliscor and Hippowdon are not the greatest of matches with very similar typing.
At least Skarm takes Ice neutral and Water is a resistance.
Hippodown can cover its electric weakness and to some extent fire weakness very well.
So if your wanting a defensive phasing core Skarmdon is the way to go but with Gliscor you can pull off all sorts of rolls.
It can taunt, poison, boost, wall and its typing is brilliant though 4x ice weakness paired with terrible SDef isnt great.
Overall I would say Skarmory is the better Pokemon really as the typing is brilliant and Def is insane.
It has the ability to work with sand teams and synergise while setting up hazards.
Though Gliscor can preform a very nice Tank set, but tbh imo they are hard to put next to each other as they serve completly differant roles.
If you will use Gliscor I suggested a set earlyer in chat, if thats outdated ask for another lol