When I look at a scenario I try to decide whether this is one that I will enjoy. if it is a style of action that I like I start to look at it from the view would I want to play either side - if that is good than I suggest it to my opponent. This is not foolproof, I have played scenarios that look good and I would take either side just to find out that they either were not balanced or had little appeal.
I agree that fun is more important, especially if you going to spend several hours playing the scenario. You definitely want to enjoy your time playing it.
I try not to judge a scenario based on ROAR W/L record unless it looks really bad (11-2, 25 7, etc) but I will click on the scenario to view the rating that players five the scenario. That being said this is subjective in that I wonder how many people give ratings below 5, I know that I will and so will my opponents. However, for making a decision I weigh that more than the W/L record. I have played scenario where I believe that my side had little chance of winning, but enjoyed the scenario immensely since it was fun to play.