You can use less Aqua Mira, drops or tablets, if you let it sit longer. E.g. I often put in half the dose if the water will sit all night. Note that exposure to sunlight and other factors can lower the ClO2 concentration, so you can't keep cutting the dose down and increasing the dwell time. I usually don't go below half-dose, for no particularly good reason.
The tablets and drops provide the same chemical, chlorine dioxide...there is no difference. The ridiculous "4 hour" wait time has to do with marketing strategy.
It has to do with whether you claim it handles Cryptosporodium and Giardia, or just Giardia. Cryptosporodium is harder to kill. Also may have something to do with whether you call it a "disinfectant" or a "purifier." Finally, water temperature is important, as noted in the post following this one.