I used to have an old SpaceBall 5000 and when we switched to win7 I had to upgrade to a SpaceMouse Pro. I also have a SpaceNavigator wired version at home (they just released the wireless version), so I can give you my opinion on these.
The new SpaceMouse Pro is remarkably similar to the ancient SpaceBall, but overall I think the SpaceMouse Pro is slightly better. The curved "joystick" does feel a little better than the ball, but other than that they feel exactly the same. The button layout is better on the SpaceMouse Pro - I'm definitely using more buttons than I did on the Spaceball. The buttons are easier to get to and they have some neat built in functions which I've found very useful. To me the LCD display on the Pilot seem useless - I have two 24" monitors on my desk so why would I want to look at a tiny display to do anything.
I had the same concern about the Navigator moving around but it doesn't move around at all. It has some significant weight to it - you can crank the joystick as hard as you want and it doesn't tip and it has one big circular rubber pad that keeps it from sliding. The Navigator is not bad but the SpaceMouse Pro is much more comfortable with the wrist support. Your hand has to hover to hold the joystick properly or you can lay your wrist on the desk and hold the joystick in a slightly awkward position. The buttons on the side are a little difficult to press too.
For me the SpaceMouse Pro is the best but you probably won't notice much of a difference in the ergonomics versus your old one. The biggest possible difference would be button layout / functions versus your current.
I have been using a Spacemouse Pro for the past 6 months and like it very much. I think the layout is better than the SpacePilot Pro. I do have the wired version of the Navigator at home and it works fine and does not slide easily. The 2 buttons are a limitation on the Navigator, but it is a decent lower-cost option.
I believe no matter which model you have, using 1 of them is very nice!