To start let's look at these Pokemon's typings. Garchomp is a dragon and ground type Pokemon, leaving it weak to dragon and fairy type attacks with a major 4 x weakness to ice type attacks. This typing also comes with an immunity to electric type attacks. Salamence is a dragon and flying type Pokemon with a weakness to dragon, fairy, Rock and a 4 x weakness to ice type attacks. Along with this typing on comes with a immunity to ground type attacks. So, the winner for typing is garchomp because even though it has a 4 x weakness to ice type attacks, Salamence also has that major ice weakness along with an added weakness to rock type attacks.
Next let's look at these Pokemon's stats. Salamance:
Special Attack: 110
Special Defense: 80
Special Attack: 80
Special Defense: 85
If I were to choose I would get the slight edge of stats to Salamance because Salamence just has more mixed attack capabilities. It can hold special and physical moves to its moveset set and can abuse very high base power moves such as Dragon Rush, double-edge, Hyper Beam, earthquake, and fire blast.
Finally we are on to the movesets. Both of these Pokemon have enough attacked power to do a lot of damage to teams. They have the stats to back up almost any move and type coverage that they have at their disposal. So let's look at Garchomp. Garchomp's best moves by level up our Dragon Rush, Dragon claw, and Crunch. It's best moves via TM or move tutor include Drago Meteor, Hyper Beam, brick break, flamethrower, fire blast, aerial Ace, shadow claw, Giga Impact, Stone Edge, Swords Dance, rockslide, Poison Jab, Surf, and finally protect. Garchomp's speed and attack stats allow it to move first and hit hard with a variety of type coverage and STAB. It has enough power lit to one hit KO almost any fairy type Pokemon with poison Jab and and cam usually KO most other Pokemon with either earthquake,Dragon Rush or Giga Impact. Now Salamence. Salamence best moves by level up include crunch, Dragon claw, Zen headbutt, flamethrower, and double edge and by Evolution, fly. Salamance's best TM moves or move tutor moves are Draco meteor, Hyper Beam, protect, Roost, earthquake, brick break, flamethrower, fire blast, aerial Ace, Steel Wing, Shadow claw, Giga impact, Stone Edge, and Rockslide. With the ability to use a variety of mixed attacking moves, Salamance can hit hard both special and physical wise. So, in move pools, I would give the edge to Salamence. It just has more type coverage and can hit hard both attacking ways.
After all of that information the Pokemon that you should use is the one, the only Garchomp and here is why. Salamence, don't get me wrong, is amazing because of its mixed attack stats and its great move pool but I just have to give the edge to garchomp because of its lack of the rock type weakness and the fact that it has a very high attack, speed, and HP stats, meaning that I can take a hit and hit hard on the attack. I did not want to cover egg moves or abilities because those are kind of a hassle to deal with. Both have pretty good abilities, rough skin and intimidate. So, if you could just get rough skin on Garchomp that would be the best ability for it. I didn't cover items either because it all is up to you on what you want to use these two Pokemon for but honestly I would put a life orb on Garchomp and make it a fast physical sweeper.
So my choice for Salamence over Garchomp is Garchomp in both listed formats.