True, you can make economic considerations, or you can make convenience decisions. I have Tetra AquaSafe Plus, which is 0.5 mL per gallon which is roughly equal to 2.5 'drops'. A drop is hard to define of course, but from what I've seen 5 drops = 1 mL at least with water.
It was on sale, and is cheaper than Prime to begin with so for me it was the better choice =)
For small tanks though, I'd rather buy a small bottle with a built in dropper for convenience. Then buy a larger bottle to refill the smaller as needed. I use Flourish for my plant fertalizer and had to get a 1 mL syringe to dose out the 1.66 mL out of the 500 mL bottle hehe.