There is no differance really. The attack simply misses with both of them, so it doesn't really matter anyway...
In real life, what move's effects are based on, some thing would be easier than others to avoid. In you second statement, the attack (Crunch) missed, this is because it is hard to see in a Sandstorm, and so obviously you would be more likely to miss. Avoiding is mainly found in "slow" moves that use a lot of preparing such as Hyper Beam.
The chance of missing or target evading are both tallied when veiwing a move in a Pokemon's summary, so it doesn't matter which one it is, they have the exact same effect.