Wirelessly posted (BlackBerry9630/5.0.0.591 Profile/MIDP-2.1 Configuration/CLDC-1.1 VendorID/105)
I definitely wouldn't orphan one flight's worth of points into a Cathay account.
CO points do not expire today but the program is ending on December 31, 2011 in favor of UA Mileage Plus (part of the merger) and going forward those points will have an expiry.
If you are definitely planning on putting the points into CO/UA CO is the smarter move thanks to potential lifetime status though if you are not regularly making elite status anyways that is less likely to be relevant.