Go to cluster mode. We have recently released ONTAP 8.2.5 for 7-Mode, and have no plans for future releases of 7-Mode ONTAP after that. With ADP you do not have to dedicate disks to boot the second controller, so you get better usable capacity. The partner who recommended you go 7 Mode for capacity needs to take better advantage of the training and tools we have available. This is from a modelling tool that we have available to partners -
7DOT - N1 = 8 drives (2 parity) = 19TB usable, 1 spare
7DOT - N2 = 3 drives (2 parity) = 3TB usable, 0 spares
7DOT total = 22TB usable
CDOT - N1 root ADP (3 slices) = 387GB
CDOT - N2 root ADP (3 slices) = 387GB
CDOT - N1 data aggr (11 data slices) = 28TB usable
Hope this helps!