Buck vs boost, 12V LED strip with 5V micro and 240V AC
Option B will need a bigger boost converter that can handle 2.5 Amps out (5050 strip current) and a AC-DC adapter of let's say 120% of that in power. So 2.5 A * 12 V = 30 Watts * 120% = 36 Watts / 5 V = 7 + Amps at 5V. Plus the likely minimal current of the microcontroller. 

Option A doesn't have those needs. A 12V 3 Amp supply, with a step-down converter of a few hundred milliamps at best for the microcontroller. Let's assume 250mA at 5V and 120% for efficiency. 250 mA * 5V = 1.25 Watts * 120% = 1.5 Watts / 12 Volts = 125 mA at 12V input.

Much smaller regulator is needed, as is the power supply. And that means both will be cheaper and smaller physically, while being easier to find. It's a no brainer. A 7 Amp 5V supply is not a common item, but you could find 2.5 or 3 Amp 12V supplies everywhere.

120% means 80% efficient regulator (20% power loss), and it's more likely to be 90%. My math here is all fuzzy back of the napkin but it's generally correct.

As to your question on how to test, basically just connect everything in ammeter or current sensing mode on a multimeter.