First off, why is Ignis only being offered to NXT holders? I thought ARDR was the new bad boy and THAT is what we should be holding, as NXT will become obsolete. It says in the Ignis team will be dumping MOST of the NXT for BTC, which will surely crash the price of NXT.
NXT is only the platform for the ICO. And in October/November when Ardor starts all NXT holders will get 0.5 Ignis.
When will NXT be extinct and ARDR becoming the main token?
NXT won't be "extinct", the blockchain will continue to run.
Also, WHY Ignis? What's the use case for this child chain?
Regular payments. ARDR isn't thought for that, it will have high transaction fees and is thought mostly for forging. You should really read a bit about Ardor.
If you want all the details read the
NXT 2.0 overview (NXT 2.0 was rebranded to Ardor).
So the question remains, should I be selling my ARDR for NXT to buy Ignis? Or should I be holding and buying more ARDR?
That depends if you want to participate in the ICO in the next rounds. If yes, then you should buy NXT, and if not, then you should tend more to ARDR as the price of NXT, probably, will fall towards the end of the ICO. You can also stay out and speculate on lower NXT prices after the end of the ICO, buy NXT for a lower price and then participate in the snapshot.