Adding a 4th Drum

I'm thinking about adding a 4th drum to my set of LP classics. I have a tumba, conga, and quinto. Would you had another tumba or conga, and why?
The World of Percussions
http://www.mycongaplace.com/forum/eng/
http://www.mycongaplace.com/forum/eng/viewtopic.php?f=13&t=7836