If you're a Cloud member then upgrading to CC is included in your membership.
It's really up to you if you want to upgrade.
But you can have both installed at the same time. If you already have Photoshop CS6 and a full CC subscription, it would be daft not to add Photoshop CC so you can try its new features.
It's not either/or. It's both/and since CS6 and CC install into their own respective folders on your hard drive so you can have both installed.