Cathay Dragon is an affiliate of Cathay Pacific, pretty much like relationship between American Airlines and American Eagle.
-Yes, it is a good airline. The food on KA is somewhat superior to CX but they are getting close. KA offers the same seating as CX regional on all aircrafts except the A320. Reliability wise because KA flies to mainland China it is somewhat more prone to delays.
-Service is similar. Maybe CX crews are more experienced but I usually find KA crews more cheerful.
-KA has older planes, but they are thoroughly refurbished so you can't really tell their age. CX is maintained by CX while KA is maintained by HAS, a subsidiary of CX, which is similar IMO.
Summary: Go with KA if it fits your schedule better. You can't go wrong too much with a regional flight...