I recommend maxed fortune as it saves time when mining, however silk touch is good for building and a great source of stone. The best thing to do it invest in both in my opinion, you could silk while mining then fortune any ores you come across, but really it's down to what you prefer.
silk touch is so much better for when you go mining though as you can just stack stuff so much better, fortune it all when you get back to your house
Silk is only useful if you want tons of stone without smelting them from cobblestone or for ores, if you got no fortune III and want to fortune it later or sell in ore state for better price. While Fortune III is extremely useful if you need alot of coals, redstone, lapis, diamond, emerald and quartz. In fact you can even use the fortune III axe on (some) crops for more yields. In my opinion, they both worth the same (I think), but in different ways.
