I am going to purchase a few chassis fans (12cm) for my computer and the salesperson told me that a fan with DC12V 0.18a has higher RPM than one with DC12v 0.16a.
I don't know anything about electronics or voltage, is what he said true?
First, there's no standardization for how fan currents are specified. For example, One might specify the current under blade load and one might specify the motor current with no fan attached. One might specify the current at peak speed and one might specify the current at average speed (for fans that change their speed). One might specify the average current for the line of fans and one might specify the absolute maximum the motor is rated for. One may specify peak startup current and one may specify average running current. You get the idea.
Second, fans vary in blade design. Some spin at a low RPM but use more blades. Some spin at a high RPM but use a lower blade area. A fan with a high RPM may draw less power than a fan with a low RPM because its smaller blades produce less air friction.
Third, designs vary drastically in efficiency. There's friction in the bearings. There's power loss in the driver electronics. And there's a question of how much of the power can be recovered when the magnetic fields collapse -- the fan motor builds up and tears down magnetic fields as the fan spins to keep pushing it, and they vary in how much of that tear down power they recover versus how much they convert to heat.
Typically, the current ratings you see on a device are the maximum it is going to draw, not the actual amounts you are likely to see. So they're not really useful for anything other than making sure you have enough power to run the device.
A superior fan design may draw the same current and move the same amount of air but run at a much lower RPM than an inferior fan design. This can be accomplished by using more blades, thicker blades, or more aggressively twisted blades. This is a common design feature of low noise fans.
So, basically, the salesperson does not understand that a fan is not an incandescent light bulb.
All things being equal yes that would be true. In electrical power (measured in watts - W) is voltage times amps. So a 12v, 0.18a fan would use 2.16W and the 12v, 0.16a would use 1.92W. So like a higher wattage light bulb is is brighter a higher wattage motor would probably produce more power and turn the fan faster.
Of course all things don't have to be equal. The fans could have different designs, the fans could be different shapes or have different weight blades or have different bearings or have more or less efficient motors.
In reality the fans will list their RPMs, CFM, and dB level so you compare those specs instead - the amperage of the fan is secondary.