Great Question! That depends on your margin levels.
However, from a value perception perspective, if you have a minimum amount spend rule set up, you may want to consider the below:
A percentage discount off orders under $100 off will appear to be of a greater value than the dollar amount discount. For example: 25% off of $75 appears larger than $18.75 off of $75.
A dollar amount discount off orders over $100 dollars will always look larger than a percentage discount. For example: $75 off of $300 appears larger than 25% off of $300.