OBP is obviously of value, but you have to put it into context. Frank Thomas OBP of .430 is far less valuable than Rickey Henderson OBP of .430. I feel like raw SLG% is not like that. If a guy has a .500 SLG%, fast or fat and slow, he is still getting those bases, so his value is the same. If I had to choose one stat over the other, my choice would be slugging.
A truly GREAT offensive player has a good number in both categories, so (overall) Rickey sorta wins that over Mattingly throughout their careers Rickey walked AND hit for power, whereas Mattingly did not draw many walks.
Career wise, Mattingly actually has a .830 to .820 advantage over Henderson in this category, but his is heavily weighted on Slugging. Rickey was pretty high in both categories throughout his career.