Nxt is making a true decentralised p2p exchange.
Ripple is centralised through few ledgers. Ledger owners can make a cartel and scam people.
It seems like you have no idea what you are talking about... "Ledger" is the balance of all accounts, orders and trust lines on Ripple, there is no "owner" - it is (like the satoshi block chain) a single distributed database that has a globally agreed state from time to time (in Bitcoin ~every 10 minutes via PoW, in Ripple every ~10 seconds via Consensus).
What you probably mean is "validators" (the nodes used to reach consensus). Yes, just like miners in Bitcoin, validators can theoretically form cartels.