Who has the better design?
Both are really cool Pokémon in every way. However, I have to hand it to Salamence. He looks truly menacing and draconic, and was the first to fit that criteria among Dragon Pokémon, alongside Rayquaza. Garchomp as a landshark dragon thing is epic, but can't compare to Salamence as far as I'm concerned.
Who would win in a battle?
Assuming both come in at the same time, meaning Intimidate kicks in, Salamence might have some kind of chance with Outrage. In most cases, Garchomp's marginally higher Speed would decide victory, though.
Who is more useful?
Both are useful to destroy Teams and kick ass, and are among the best Pokémon ever, so I'd say they're equal. Garchomp has less "bad" points, though. Ground Type makes him resistant to Stealth Rock, immune to Sandstorm, and get Earthquake STAB, all of which are epic things. Salamence is weak to Stealth Rock. Both are excellent, though.
Who do you like more?
Salamence. I prefer his design, he's an astounding force, and is my 4th favorite Pokémon nowadays. Garchomp is cool, but just not as cool as far as I'm concerned, so I concede my vote to Salamence.
