NXT and Monero, these two are completely different coins and they cannot be compared IMO.
Which one has a better chance of becoming a successful alternative to btc/ltc?
I'm sure Darkcoin or(and) Zerocoin/Zerocash will overcome Bitcoin someday.
Why can two different coin technologies not be compared? What are the advantages and disadvantages of each tech?
Monero is maintained for anonymous transactions but it has some flaws in its code and other problems like being botnet raped.
NXT is the first pure PoS coin written from scratch. It brings asset exchange and some other features but suffers from unfair distribution.
Which one is better and how to compare them is up to you.