I'd suggest using the Spaceball because the ball makes it easier to control spining and zoom operations because of how it fits your hand. Also it allows the use of 12 function keys which is more than the Spacemouse.
I've never used the space mouse but I'm sure either one you choose will help you perform modeling tasks much quicker.
We had a spacemouse and only one designer preferred it over a Spaceball 4000. We have 44 4000's and 5000's for our 75 workstations and laptops that have CAD on them.
Some people just don't really need it and some just don't use them. Some users said they didn't think they would use, but we put one on their desk anyway, and a month or two later they tell me I can't have it back.
The Spaceballs have more functionality with UG than Pro/E, but with Wildfire, I don't reach for it has much. The 5000's are nice because they have a MSOffice driver that lets the Spaceball zoom and pan Excel and Word.
"Wildfires are dangerous, hard to control, and economically catastrophic."
I would lean towards the Spaceball. I work at two difference workstations which use the Spaceball. Not only are they nice to use, but they have also helped improve productivity. I use mine on CATIA, UG, I-deas, and Pro/E. I makes for a nice universal standard for CAD display manipulation.
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