You plan to stay with NetApp for the long term. Data ONTAP 8.2.x is the end of the line for 7-mode and all new capabilities are going into CDOT-only releases.
You plan to expand your NetApp FAS infrastructure with additional nodes. CDOT lets you scale your storage capability across multiple FAS nodes in a much more seamless way than 7-mode.
You want more flexibility over the management of your physical storage resources. CDOT has much more robust non-disruptive operation capabilities than 7-mode, such as head upgrade, aggregate relocate, volume move, etc.
Here are a few things to consider before going to CDOT.
You will need to transition to a new CLI environment. Some like the new CLI, others miss the 7-mode days. Not all of the 7-mode capabilities are available in the CDOT CLI, but the gap is narrowing. The new CLI is much more structured and regular.
Transition to CDOT will require additional hardware, aka "swing gear", since your 7-mode aggregates do not convert in-place to CDOT. Ask your VAR if they have such gear available. Berkeley Communications is one such NetApp partner who has this swing gear available for their customers. NetApp sales reps used to lend out demo or evaluation gear for this purpose but NetApp corporate has clamped down on this practice lately.
The LIF concept in CDOT, which virtualizes the network layer, will take some getting used to.