This is a hard one to verbalize, but seems easy to demonstrate.
All I can think of is that in a Hard Style, you sweat a lot more.
I'd like to see how others who experienced both soft and hard styles verbalize it.
I study a very soft style, and we don't really talk about or study about Ki at all.
Tough question, because everyone might have a different point of view based on there size, age and experiences.