BC is essentially just a bitcoin clone with a very fast PoW phase so now the mining has finished. Bitcoin will be like this someday too in a long, long time. But as far as what's under the hood, they are the same and do the same thing. There isn't anything that BC can do that bitcoin can't do and will adapt if needs be.
NXT on the other hand is a whole new platform, not just a coin. Right now, it is only a coin, alias system, and messaging system (already much more than BC), but an alpha asset exchange has already been released and is being tested. On top of that mulit-sig is being worked on. This has already had its proof of theory tested and proved successful but is still in the very early stages. One developer has succesfully transferred Doge into the nxt ecosystem, had it represented by NXT and then had it safely transferred back out into Doge. This isn't easy to do at all or many people would have already done it. (I am talking about you Mastercoin). But if/when this is fully automated and working properly it will be the killer app. That basically means NXT will be a decentralized Mt.Gox, except it can't fail. No trust in a third party, no more wondering if an exchange will run with your coins. While you make an exchange, the coins are always yours. On top of that, the fees are planned to be much lower. Comparing any current coin and NXT is kind of silly. Ripple could be compared but has its flaws too, and maybe in the future Ethereum, but....... Ethereum is in development and NXT is here now.
BC is just another bitcoin twist marketed and hyped to be something really cool and special, but is really the same old thing.
Truthfully, we don't know what exactly NXT will achieve. Its flaw is that its original IPO didn't have that many investors. At the same time, its IPO was open for a very long time, anybody could have joined, but 99.99% of people thought it was a scam. People constantly harassed the developer saying he was just taking the BTC and running. In the end, he gave the community the most interesting surprise since bitcoin, and out of nowhere.
People said bad things about it because the IPO was so small. But in fact, in someways that is also its strength. A few hundred people ended up being big holders after the first few weeks, mostly big whales and nerd types. The whales are giving out huge bounties to anybody developing the code, and the people that can actually code (not just copy and paste), are working really hard to develop it. In this way a nice community has been built by people that won't have anything to gain unless NXT is very successful and to that end NXT is being developed quite well. If you would like to have a look, please click here and try out a test network beta of the next nxt client.
http://nxtra.org/nxt-client/Or install the current real client
https://nxtforum.org/nrs-client-how-to-help/(nrs)-(0-8-12)-latest-nxt-client-and-english-tutorial-(2014-03-21)/ and get some free coins from a faucet here.
http://nxtra.org/faucet/As of right now, NXT is more or less just another coin with a very green mining system and a few options. Not actually that much different from much that is already out there, in six months from now if nobody is able to write code, it will still just be a coin with a few more options than any other coin right now, but if the developers come through, it will be far superior than anything the community has. Seriously!