I believe the first Glaceon is the better one to use.
I'm assuming you want to use this for the PWT/Battle Tree/any other in-game Battle facility. In this case, the first Glaceon is the better one to use.
Glaceon has good defense stats, which means Choice Specs is more helpful. It can tank a hit and fire back with more power than if it was holding a Choice Scarf. Glaceon has a measly 65 base Speed anyway, so it would still be outsped regularly.
Modest Nature is better than Timid nature in this case, as the first Glaceon is going for power over speed.
If you're going to keep the first Glaceon, I recommend replacing Mirror Coat with Hidden Power. It doesn't matter what type as long as it's not Ice or Ghost. Having Mirror Coat on a Choice Specs Pokémon is wasting potential. Unfortunately, Glaceon's special movepool is rather limited, so you can either keep two Ice moves (Blizzard and Freeze-Dry) or replace one of them with Round or Hyper Beam. Not many good choices. If you tell me which game you're playing, I can recommend some move tutor moves. Edit: You said you're playing SwSh, and Glaceon can learn Hyper Voice by TR. I recommend replacing Mirror Coat with Hyper Voice.
And finally, don't use Synchronoise if you decide to go with the second Glaceon. It can only hit Ice types.
As for your comment, Assault Vest is probably the better option. It buffs Glaceon's Special Defense but only allows the use of Attacking moves. The set you commented has only attacking moves, so it works well. If you want to use any status moves, such as Hail, use Leftovers.