I'll echo your experience. I'm using MotionX GPS HD on my iPad for hiking and city walking; and MotionX Drive HD for driving. They are both superior GPS apps. I too have noticed that the iPad holds a signal better than my handheld GPS, an old Garmin eTrex. I also note the absurd Fastest Speed numbers in MotionX.

I'm jealous of your iPhone 4S! The iPad is my first iOS device but I see an iPhone in my future. I have heard that the iPad GPS is more sensitive than the iPhone's. But it would be a lot easier to take hiking!