You don't get ARDOR anymore with your NXT. That snap-shot is already over.
You will get IGNIS. It's the first child-currency that runs on Ardor.

So basically: If you had NXT some months ago, there was a snap-shot and you got ARDOR assets. For each asset you will get one ARDOR on the final chain.

Now there is an IGNIS-ICO, you can sell you NXT into it. If you're lucky, then the offer is taken and you get roughly 1.32 assets which represent IGNIS (not all offers are taken up, because there are too many).
There are still more rounds to come (all information is collected here). But if you don't take part or don't succeed, then you just keep your NXT and you will get 0.5 IGNIS additionally on the day of the launch.

What's the best deal? Hard to say. If you trust in the new launch, then you would want more IGNIS. But it might also drop in price... or NXT might go up, and then maybe keeping NXT would have been the better choice. People might also dump NXT in the thought that Ardor will take totally over. So yeah.
Either way, with all the money raised so far, the devs have given the guarantee that Nxt will be supported for 3 years at least. And since it is different from Ardor, there are still good use-cases for Nxt.