The advantage of the SpaceExplorer and SpacePilot are that your hand rests on the controler, holding it firmly on the desk. I do know someone that made a base plate for the SpaceNavigator that did much the same thing.
I'm not trying to put you off the SpaceNavigator, - it's a great bit of kit for the money.
There are old type SpaceBalls for sale on ebay, but you have to be very careful if you consider one of these, as the older serial type connection may not work on new workstations - even if they have a serial port. It's to do with Intel (and Microsoft ?) not providing full support for serial devices in the new chipsets - some work, some don't.
I'm not sure about older USB spaceballs.
Whichever you decide on, if you ever use a worstation without one it's really strange.
bc