There are theoretical merits to each approach.
Longer singles should force you to cycle through the different sources of carbohydrates available to you (either in your liver or in your muscles) more frequently, which, in theory, should make you more efficient at doing that. That efficiency should have a positive effect on race day if you are running a marathon.
With doubles, because certain hormones (I honestly forget which hormones) are released only during the first twenty minutes of exercise, you get the benefit of having certain hormones being released twice per day instead of only once.
Personally, I have always been a lot of long singles kind of guy because I like the mental adaptation that comes along with thinking of 10-12 miles as an easy recovery run. I think there is merit in that mentality when you are racing the marathon. Also, less laundry, less transition time, etc.
But if you aren't running the marathon, or if it is only happening here or there during the training cycle, I don't think it really matters very much at all.