LR CC has extra capabilities compared to LR 6 — specifically it has Lightroom Mobile and Photoshop CC and free upgrades as long as you subscribe.
Only you can decide if you NEED Lightroom CC with the extra features that LR 6 doesn't have.
Lightroom 6 upgrade is 79 US Dollars (if you are not in the US, you will have to look up the price yourself at Products, scroll down to LR 6, click on Buy, change "Full" to "Upgrade")
CC owners will also receive new and enhanced feature updates from Adobe as soon as they are released.
LR6 owners will have to wait until LR7 then pay to upgrade.
e.g. A new dehaze tool is imminent. No extra cost, immediate update for CC owners. Expect LR6 owners to get upset.