Also... Stuart's RAMcC came at a time when the south had been suffering reverses on the battlefield (Seven Pines, Forts Henry and Donelson, and I think Norfolk had just been captured). It was a huge morale boost for the army and the civilian population, as well as successful in intelligence-gathering. Also he lost only one man.
That said, I was also tempted to vote for Grierson because it was such a big part in enabling Grant to get across the Mississippi. In its ramifications/fallout, it's probably the more important of the two. But as far as the rides per se, I voted Stuart.