As long as you cast them both in relatively quick succession (I believe you have to cast it within 0-.25 sec of each other, if I remember correctly) then either method will deal the same damage.
I actually remember making a topic just like this one a year or so back. This thread pops up quite often actually
Anyway, with the MECH (damage) out of the way, here are the STRATEGIC benefits:
-ava then toss will end up disabling the enemy slightly longer than toss ava
-has the potential to miss ava entirely
-ava has a longer range than toss
-you can't really miss toss, unless there's more than one unit beside you
-toss first guarantees that you'll be able to hit the ava too, for the combo damage.
-disables the enemy for a slightly shorter duration than ava toss
-toss has shorter range than ava so against a farther opponent you will have to ava toss
-might grab the wrong unit
Overall I suggest using toss-ava when you can since it's a guaranteed 100% hit on everything. However, you will usually HAVE to use Ava-Toss because enemies will run when they see you. It's unlikely that you'll actually be able to constently get within tossing range.
In my experience as Tiny, my combo will usually involve running at an enemy and throwing avalanche at them. While the avalanche is still flying and stunning them and any nearby creeps, I run to their side, away from the creeps, so that I am 100% sure I will toss my intended target.
If they are all alone, go for toss-ava-start attacking them with your normal hit.