both are good, i hold or have held both in their respective wallets and not just on exchanges. both strong communities (although i think ARK, with delegated proof of stake has a stronger community)
you need a lot of PIVx to get noticeable staking (~10,000+), and you need to be staking 24/7, but with ARK you can purchase a lot and get daily or weekly free ARK depending what delegate pool you join, even when you're offline. Of course the amount you get is small, but it's a good feeling to get free ARK every day
we're talking ~0.1% a week staking, which will diminish slowly over time as ARK community grows.
so i say both are good, but if you have money to blow, and you have to choose, give ARK a chance