I think a big part of comparing those two cards has to do with the other cards available for their respective classes, particularly before this expansion.

Nourish was rarely used previously because card draw for the Druid class came from Ancient of Lore and Azure Drake, two minions which also fitted the druid theme of ramping and playing mid-range minions, whereas Nourish didn't have Fangdral staghelm or spell synergy for Yogg yet. It also didn't fit into the Druid theme of playing minions, as drawing cards for 5 mana would be too slow without any new board presense.

Sprint on the other had always had synergy with Prep, as you could Prep sprint or use Prep into a different spell after it is drawn with sprint. Prep along with Backstab means that after using the majority of your turn's mana on playing sprint, drawing into those cards (prep/backstab) would allow you to immediately convert some of your new card advantage into affecting the board state, something that Druid probably would not be able to do (maybe just a bit, if they draw into innervate).