Just one thing to consider if you tour in hot places...
I have found bottles stayed much cooler in panniers than in external bottle cages. I tried the many bottles in cages thing and hated having all very hot water. Since I figured that out I started carrying any extra water in the panniers. The two bottles in the cages still get hot but at least when I stop for a break I have some cooler water. I don't mind stopping and refilling every time I drain two bottles (either from bladders/bottles in my panniers, from a store, or filtered from a mountain cold stream).