My experience would be that CO is better than DL. I have not had any problems redeeming from YVR on CO, flexibility helps. I tried redeeming YVR-LHR using Skymiles last December, they could get me there on CO but returning on DL to Cincinnati on to NY then stuck. Onepass got me the same flights to LHR on CO, the same DL flight to Cincinnati then home on DL via SLC. I even booked 3 reward tickets to MCO without any problem using Onepass. I also find the service a lot better on CO vs DL. Just my personal experiences over the last 10 years.