That question can't be answered. We don't know the price of NXT in the future so we do not know if investing in the ICO (first round = 2.5 IGNIS per NXT) will be more worth than holding your NXT for the Snapshot (NXT + 0.5 IGNIS per NXT holding).
You have to decide for yourself... only time will tell.
My advice, both is correct:). I think that by the end of Ardr launch both ways will be more or less worth the same.
RE: Change the NXT to IGNIS or hold NXT for free IGNIS