Jillian is perceived to be tougher, but now and then Bob will say something to someone that I feel is out of bounds. I think that even if it was 20 years ago, Jillian has been obese and so she can empathize to a small degree with the contestants.
I guess the instance that comes to mind is something Bob said to Shay, I think that was season 8. They way in which it was judgmental was very subtle, but it came across to me that this is a guy who has never been fat.
The teams are generally chosen by someone who wins a mind game (or, really, loses one).
I've started watching Losing it with Jillian Michaels (it's on youtube) and I like how she helps people declutter as well as work out. It wasn't so entertainment driven, I guess that's why there was only one season. She probably crosses the line into psychotherapy, and I have mixed feelings about that.