I thought proof of work was a better way for decentralization than pos
It depends.
Do you consider the current mining landscape in Bitcoin "decentralized", with one pool/mining equipment manufacturer (Bitmain with Antpool, ViaBTC etc.) dominating about 50% of the hashrate?
So centralization can happen in both algorithm types. What your probably mean is the "rich get richer" feature of PoS, because "the rich" can get minting income. However, in Nxt/Ardor this income is pretty small as there are no real block rewards, only transaction fees can be "forged".
In Ardor this "rich get richer effect" will be probably a little bit stronger than in Nxt, because of the high transaction fees on the main chain and the fact that every child chain "bundler" must pay the fees to include child chain transactions in the main chain. But "leased forging" (=pooled forging) would also be available, so smaller accounts could also participate.