Let's look at the different advantages of these Pokemon:
Altaria is a well-balanced Pokemon, stat-wise, with decent stats in everything... UNTIL you mega evolve, then Altaria is EVEN BETTER. You thought I was going to say something bad there!
Mega Evolution. Altaira has the upside of Mega Evolving.
2 Words: Cotton. Guard. Cotton guard is AMAZING! Thats the move that combined with good stats and mega evolution, makes Altaria a TANK!
No more being killed by dragon types! Altarias mega-evolution is dragon and fairy so Altaria is nowhere near as vunerable as before!
Small option of moves. Altaria doesn't learn much by level up so you'll have to get some good TM's.
I seriously cant find anything else...
Salamence is also very well balanced, and he's quite over-powered with his mega-evolution!
Mega-evolution. Like I just stated, his mega evolution is over-powered! But that brings some downsides too, as mega Salamence is stuck in ubers!
Good moves. Unlike Altaria, you don't have to worry about not having good moves for Salamence!
Psudeo Legendary. He's one of them because he's one of them; really, really, strong!
Type weaknesses. He's got more typing problems than Altaria because he's the same as every. other. dragon. type...
He's predicatble. Like, really predictable. Like, Yep it's Salamence... Yep. Uh-huh. Just use Ice Beam, predictable!
Like I mentioned above, his mega-evolution is a uber, so if your battling compeditivly, you cant use mega-Salamence in lower tiers.
And like Generekt pointed out, Salamence is really hard to raise.
Now how about some movesets?
You asked for moveset tips, so here's an idea for some movesets for each!
Dragon Claw = STAB, reasonable power, good for smashing those other dragon types you meet!
Flamethrower/Fire blast = Good for killing those ice types that want to kill you, and more!
Roost/Fly = That move you use when your low on health, and it drives most people insane! And Fly will work best here, because you said a Pokemon that can learn fly
Double-Edge/Hyper beam = The most powerful moves that Salamence knows, they're just powerful, you cant hide it!
Moonblast/Fly = Powerful, and it works great against dragon types! Also, mega-Altaria would have STAB. And you said you wanted a flying type that could use fly so...
Sky attack/Fire blast/Flamethrower = Sky attack is a good powerful attack, but Fire blast or flamethrower will deal with ice.
Cotton guard/Dragon dance = Dragon dance is a good stat boosting move, but Cotton guard is an absoloute no-brainer!
Roost/Fly = If your opponent actually does decent damage through your Cotton Guard, Roost will refresh your health before you go down. And this is another place where you could have fly.
Hope I helped with your decision
...But if your wondering, or still can't choose, My choice is Altaria!