Samence is, without a doubt In my mind, better. At least in my experience.
Salamence is bulkier, a MUST in my opinion for ranked battles.
It's also faster than Mega Dragonite, which means easier sweeps.
Salamence is also way earlier, with Salamencite only being 360 Mega Shards. While Dragoninite is VERY late game, which probably doesn't matter because you are asking about both, which likely means you have both, but it still stands for playing in general.
Dragon Dance boosting until you can clean sweep anyone is also just insanely fun. (But that's besides the point)
And for a bit of speculation,
If and when they put it into a game with abilities, unless Mega Dragonite gets Huge Power or Speed Boost or something (which to be honest, isn't too far-fetched), Mega Salamence's ability is better. Aerilate is a busted ability. But again, it's when and if, if being the key word. We might never see Mega Dragonite again (Doubtful, tho)
Hope this is helpful. Also hopefully you get Greninjite if you haven't already.