That really depends on your design. If you have say, swappable panels each with an optical device or two then the optipac would be the way to go IMHO. If you are building a fixed panel that includes one trackball, ps/2 is easy and cheap, so why not?
As for Betson vs HAPP, there are a few options there too. Betson sells a non-ps/2 trackball as well. Both can be found quite often on Ebay at very decent prices. Hack off the molex connector of the non-ps/2 model and you can hook it up to the opti-pac pretty easily. If you can only get your hands on a ps/2 model and you still want to go the opti-pac route, you can hack it to bypass the ps/2 stuff.
Hope this helps.