The soft serve might also be frozen yogurt which has natural good bacteria which again helps digestion.
The "real" ice cream might have enzymes that processed ice cream does not. The enzymes in the real ice cream might help you digest the dairy better. Can you eat cheese without problem? Can you eat yogurt without a problem?
While soft serve type ice cream is not health food, I would say it could be part of a healthy diet if it is a treat now and then. I strongly advise you to avoid the types of dairy that cause you discomfort and be aware this may progress to where you have trouble with soft serve also. You might consider trying almond yogurt - my boyfriend loves it. I find it at Whole Foods.
He and I have both had to give up most dairy.