It depends on the Pokemon and their maximum/minimum stats.
IVs are a hidden value out of 31, so at level 100 a max IV Pokemon would have 31 points more than a 0 EV Pokemon. Natures on the other hand increase or decrease a stat by 10%.
So if the stat at level 100 would normally be 250, then the nature would add or remove 25 points. In that case, good IVs make a bigger difference. But if the stat was around 400 normally, then 10% is 40 points plus or minus, so the nature makes a bigger difference.
In short, IVs matter more if the final stat is under ~300, and nature matters more if the final stat is over ~300.