Use decimal whenever you're dealing with quantities that you want to (and can) be represented exactly in base-10. That includes monetary values, because you want 2.1234 to be represented exactly as 2.1234.
Use double when you don't need an exact representation in base-10. This is usually good for handling measurements, because those are already approximations, not exact quantities.
Of course, if having or not an exact representation in base-10 is not important to you, other factors come into consideration, which may or may not matter depending on the specific situation:
- double has a larger range (it can handle very large and very small magnitudes);
- decimal has more precision (has more significant digits);
- you may need to use double to interact with some older APIs that are not aware of decimal;
- double is faster than decimal;
- decimal has a larger memory footprint;
