"If you decide on a number of times through and actually count, you can stick to that pretty well, but if you decide a running time, it is much more difficult to stay on track of the time and warn the band three times before you want to end, etc."
I couldn't disagree more. I have always used a stopwatch to keep track of how long I'm running the dance, and since one time through the dance is close enough to 30 seconds as to make it not matter, I signal the band 3 more times at whichever B2 is closest to one and a half minutes (usually 6.5 minutes since we started) from my target time (usually 8 minutes). I don't count times through or use marker couples or anything like that because I don't need the distractions. If I were counting, I would definitely have a moment where some couple needed my help and I'd be figuring out what calls would best put them back on the right track and forget whether I was on 5 or 7, and then I'd probably end up running the dance too long thinking I was on 16 times through, when it was really 20.
If counting works for you, that's great. For people like me, having a target length is much simpler.