comparing 2 airports
The main question you need to ask yourself is how you feel about travelling with crowds.
The simple issue that BRS has far more flights and services with LCC operators means you will get good choice with excellent prices for flights.
This however will drive costs in the terminal up which will result in higher retail costs. Also it will be much busier and hence you are likely to have a longer security wait. Depending on time of day.
At CWL, the airport is still a long way from nearing filling its capacity and hence is actually much easier to travel through. The security lines are seldom longer than 5 minutes apart from Thursday early mornings which are probably the busiest time.
I prefer to use less busy airports and like to be treated like a true customer rather than simply another number. Cardiff for me is always a better option as I get this experience every time, while at BRS, I seldom get greated with a smile, always had to wait in long lines at security and for some unknown reason have to wait even longer to get back into the UK at passport control due to a crazy system.
I also found the lounge much better at Cardiff as this is run by the airport info staff almost as a treat. They put loads of effort into ensuring we are looked after in the lounge if needs be. While at Bristol I have been grunted at on a number of visits when just going into the lounge.
Hope this gives you a bit of food for thought.