They are both nice but I think Costa Linda is nicer. We stayed at CDM last year in a two bedroom and while it was very clean and comfy is appears a bit older. Have seen the Costa Linda rooms and they are lovely. The unit at CDM was an exchange and it had a great view of pool and ocean so if it came to view that could decide it. I think all the 2 bedrooms at CDM are ocean view but the one bedrooms are across the back parking lot on the street if I recall. That would certainly make a difference to me. I will say we exchanged with RCI and the info listed on the confirmation was the unit we got if that helps. Linda is the local Costa Linda expert so maybe she will chime in.
CDM appeared older because it is older. Both resorts were built by Sun development corporation which first built ABC, then CDM, they then built Playa Linda, which we bought into in 1992 at a close out sale because they were selling their newest resort Costa Linda. After Costa Linda they built the golf course with villas at Tierra del Sol.
I believe that Sun development then went bankrupt but I'm not sure about that. I do know that both Playa Linda and Costa Linda voted Sun development out as their management company many years ago. Playa Linda then hired their own management company which has been doing a great job running the resort since then.
Costa Linda recently removed their Board and Management and voted in a new Board who have a temporary agreement with Royal Resorts to manage the resort. The Board is now considering a long term contract with Royal Resorts.
Probably a lot more information than you asked for but that's a little history on timeshare resorts developed by Sun Development in Aruba.