I have experienced terrific service on CO from ground and air personnel, both internationally and domestically. That said, as noted the DL systemwide upgrades could be valuable if you travel internationally. IMHO, DL's coach international service is far inferior to CO's, in both comfort and level of service. I sadly can't comment on the BusinessFirst/BizElite, though the seats are certainly appealing.

My sense is that DL also has more restrictive blackout dates on awards, if this is at all important.