This is just my thoughts:
-you should plan to fit a 3" arcade ball into your cab if you have the room, even if that isn't the route you are going right now. In six months your favorite game might be a TB game and you want to upgrade. If not, you didn't lose anything.
-If you are on a budget, go with the iBall for now (remember to leave room for the big daddy 3" when you get the money.
Why go for the iBall now? Because it's a nice looking ball, you can get one for about $9, and you can hack it as an iterface when you upgrade to the three inch ball later (again, it's more work to hack your own interface, but if you are on a budget...). If you don't want to bother looking or can't find it for $9, PM me and I'll sell you one.