Optipac: Connect up to two trackballs and 4 spinners at the same time. Can use 2 TBs, or 4 spinners, to (if wired correctly) 1 TB & 2 spinners at the same time. Each TB can have two buttons, and each pair of spinners can have two buttons. Buttons follow matching device combos, so a max of 4 buttons can be used at a time. A little more than needed if you're only doing one TB.
Mini-Pac: A mouse + keyboard encoder; connect the TB & two mouse buttons, and a bunch of keyboard stick & buttons. Great for starting a new CP, a little more than needed if the joystick & buttons are already connected and you're only doing one TB.
Optiwiz: 3 mouse axes and 3 mouse buttons. Probably more up your alley if you're adding the TB to an existing board already wired up. But...
A) Did you get the USB/PS2 trackball? If you did, you don't need any of the encoders.
B) U-HID Nano: Very flexible. Can connect the TB and 6 buttons, or TB and 8-way joystick and two buttons, or TB & analog joystick and 4 buttons, or TB & spinner & 4 buttons. The buttons can be mouse, joystick or keyboard (with max 3 mouse buttons). Might be more than you need ATM, but with its flexibility it might be more useful it you decide to make any more changes. Or if the TB is going on a new "TB dedicated" board, might be better than the minipac.
So it partially depends on what you plan to do with the TB, and what you might do in the future. I suggest either optiwiz or the U-HID Nano if you're adding to an existing board, the minipac if you're starting a new board with TB and sticks and buttons, and the U-HID Nano if you're doing a new TB limited CP.
And let me state again, if you got the USB version of the TB, you don't need any encoder as an encoder comes as part of the TB's boards. Be aware, though; some (many?) people have had problems with the happs USB boards.