I'm not sure the level of sweat is a good judge. I do 2 days a week of a softer style Sensei (Tohei influence) and 2 days a week from a harder style Sensei (Aikikai).
I sweat my ass off at both classes! But that's just me. There are plenty of others in both classes that don't break a sweat at all. Level of sweat is really based on who your uke/nage is.
You're right that it's difficult to verbalize, but there is a distinctly different feel when practicing.