I'm considering putting 2 trackballs in my control panel. Probably a stupid idea considering the additional cost and hassle, but we're big fans of Marble Madness.
Stupid idea?! Balderdash! I think two player Marble Madness is the only reason you need to shell out for an additional track ball. Of course, I'm biased. I've wanted my own Marble Madness cabinet since I was 10. Besides, there are a few other multi-player trackball games (Atari Classics Prototype with 2-player sim. Missle Command and Centipede).
Keep in mind though, you'll lose a bit of space with two balls instead of just one. I don't know your plans but you may have to opt for switchable, modular panels (basically what I'm doing) or build a little wider. Of course, I may be wrong and you may have an ultimate solution - so I'll reserve assumptions.
To answer your question: I'd go with the OptiPac. Whammoed explained why nicely so I won't add anything else except for a tick in the OptiPac column.