Nectarines are crunchier than peaches as nectarines have a firmer texture than peaches do.
Both nectarines and peaches are of the same fruit species and do have similar nutritional profiles and similar flavors.
However nectarines have a genetic trait which gives the nectarines a smooth skin and a firmer and slightly more dense flesh which olds it's shape very well.
And peaches have fuzzy skin as well as a softer and sometimes more floral texture to them.
Although nectarines are crunchier than peaches, botanically speaking, nectarines are in reality just peaches without fuzz.
When nectarines are ripe, the ripe nectarines should be soft, but not mushy.
A ripe nectarine should yield slightly to some gentle pressure, especially around the stem end of the nectarine, but it should not feel overly soft or mushy.
White nectarines often are softer when ripe than yellow varieties of nectarines.
If the nectarine feels rock hard then the nectarine is not ripe yet and needs more time to ripen and if the nectarine feels mushy then it's overripe.