We do both. We both work and we have a son so spur of the moment doesn't happen as much as it used to. For example, even if the night isn't planned, I can promise that no sex will happen until 9pm because that's after my son has gone to bed. That kills some of the fun that we used to have just lounging around and having things progress, but it's part of life.
We do have date nights where sex is a sure thing, so I guess that's fully planned sex. In the case of date night, it's no less fun.
I miss random sex, but we don't plan every small detail either. We try to have a nice balance.