Both are great Pokemon, yet very different
Altaria
Pros:
> 1) Has an amazing mega with an great ability
2) Has great abilities
3) Has a wide moveset variety
4) Great special defense
5) Has much more resistances than weaknesses
6) Good source of moves for STAB
Cons:
> 1) Has fairly low and average stats without a mega
2) Doesn't have much HP nor health
Tropius
Pros:
> 1) Good use of abilities
2) Effective variety of STAB
Cons:
> 1) Has more weaknesses that resistances
2) Doesn't have much of a base stat total
3) Doesn't have much of a variety of moves (3 types without HM/TM)
4) Can't mega compared to Altaria
As you can see, they're kind of opposite uses. You may want to lean more to Tropius, probably because he's easier to obtain and use. Altaria - in my opinion - would be better because he's a strong mega whose great in battle (from my experience). But, it's up to you.
Possible Movesets:
Altaria @ Altarianite
- Moonblast/Dazzling Gleam (Mega STAB)
- Flamethrower
- Ice Beam
- Sky Attack/Dragon Pulse (STAB)
I'm not to lenient toward Dragon - even though is provides STAB - because Fairy can already cover dragon's effectiveness's. But, it could work too
Tropius @ Whatever Item
- Leaf Storm (STAB)
- Air Slash (STAB)
- Synthesis (for low health recoverage)
- Toxic
Tropius doesn't learn much of a wide range of moves, but I think this could be a possible moveset for him. Note: these movesets were whatever came into my head.
As you can see, I mostly like Altaria better, but that doesn't mean I didn't find any potential in Tropius. I honestly would go with Altaria, but based on your question says otherwise. Your choice, Hope I helped!