You can use either! As an American, I'd probably read such an image as "I smell the rain" or "he smells the rain" simply because it is shorter. It is a more active way of stating your or is action, while the present continuous feels a bit more passive, almost. It's a subtle difference though (more stylistic than anything).
Example where both are right, but the first option is a bit more effective
The dog hears the vacuum, so he runs away from it! OR The dog is hearing the vacuum, so he is running away from it!
Both are correct :)