Gem,
In sales you get a much larger percentage because you are working with the cost of the house. But it takes longer to get paid because the process takes an average 6-8 weeks+ from an offer accaepted to the completion date.
In lettings you get much less money for each let (you are working on much lower figures) but you get paid quicker and generally the ratio of lets to sales means you will do more lets compared to actual sales. Eg 5-6 lets to each sale. So the money soon equals out.
Hope this makes sense.
3R.x