A very abstract comparison would be that Ripple is more like the banking system where you are mostly concerned that you can send money from any account to any other account in the world.

OT (as far as I understand it) is more like the stock market, where you own some tokens that you want to trade from time to time on some exchanges. You find others who want to do the opposite (e.g. you want to sell XYZ, somebody else wants to buy), you agree on which server to trade on  (in the "real world" this is a given, since not all stocks are traded on all stock exchanges) and then trade fast and in private there.

I didn't look too much into NXT's asset exchange, so sorry - maybe someone is more knowledgeable there.