Soda vs tonic is all about flavor. Soda is almost flavorless and unsweetened. Tonic is laced with bitter, aromatic quinine and sweetened. In the better tonic waters, the sweetness balances the bitter aromas and produces a great mixer for wherever you want that particular twang. Soda is as universal a mixer as you will find due to its neutrality, shelf life and low price.  If you are obliged to choose only one for your bar stock, soda would be the one. For general mixing, stocking a ratio of 80:20 - 90:10 in sodas favour should work well. The obvious exception would be G & T lovers. There are a heap of delicious cocktails featuring aromatic ingredients such as Cynar or grapefruit to spruce up the classic pairing for when the mood so takes you.