I took both Economics (AQA) and Religious Studies (OCR) (or as we all preferred to call it, Philosophy & Ethics).
Economics is relatively straight forward, a lot of questions are binary (as in, there are two possible outcomes to an effect) and it mainly looks for exam technique and basic understanding of markets. I did a bit of extra reading but nothing major. I wouldn't say essay writing is the hinge point for the subject, it's more basic understanding and applying good technique.
R.S is something totally different in my opinion as it looks for a pretty comprehensive understanding of the arguments/theories you're going to study. It requires a lot of extra reading so really is a subject where thoroughly enjoying it makes it a lot easier. Exam technique plays a part, but isn't as important as K&U. Essay writing is also a consideration: it develops your skills sure, but being good at it already helps.
When it came down to work put in, getting an A in Economics was a lot easier than the A in R.S.