This is a tricky one because Senna was so much more popular.
Piquet was the kind of driver who's not as exciting to watch as someone like Mansell, he always spoke whatever crap he thought at the moment without thinking about the consequences, and his actions would make him look cynical. Even in Brazil he wasn't liked very much.
Senna on the other hand had a very aggressive style which everyone likes to watch, but most of all, his charisma was unmatched. Whenever you hear him speak about racing on the limit, it's a surreal experience. The time he helped Komas after the crash in 92 and his reaction to Roland's death made him look somewhat more human than other drivers. And his subsequent death elevated him to a hero position.
It's a bit hard to forget those things when making a comparison, so I'll try my best to detach from them. I watched the other day a documentary on Piquet, and how much he struggled throughout his career to get in F1. He went to Europe very much on his own, with no help from his family, and after very few years he made it to F1 when he was 30. During his career what set him apart from the rest was his ability to develop the car and set it up for the race. Things like the water brake are a testament of his ingenuity. As a driver he might not have been as fun and aggressive as others, but his overtaking on Senna in Hungary shows how good of a driver he was. And his unorthodox psichological fight against Mansell shows how mentally balanced he was.
Senna on the other hand was massively fast with an astonishing raw speed, but in his first years he would have lots of ups and downs, with a lot of crashes, and many aggressive maneuvers that made other drivers in the grid not like him so much. During races, he would rely much more on his raw speed than on having a conservative approach of saving the car and the tires. But as he grew older he started avoiding those early silly mistakes, and became a much more cerebral driver. The problem is that when he reached maturity, he had an uncompetitive McLaren to race with. It's a shame he died, because we couldn't really appreciate what he could do with a top car at his peak performance.
After all, Piquet was a more complete driver, but Senna had the upper hand on speed.