The best way to clean a computer is neither with the vacuum nor the air gun.
The best way to clean a computer is to use both.
For example, if you are cleaning your Power Supply then hold the vacuum at the output fan and use the compressed air on the inside opening/fan of the PS.
After you are finished with that, then reverse the process putting the blower outside and the vacuum inside.
Changing the direction also loosens and removes any dust that might have gotten lodged somewhere. After you think you have it all get the odd angles with the blower and you'll be fine.
The biggest risk that you have in this process, is with getting particles into your eyes, so be sure to wear goggles. Glasses help but, are not enough; the particles will get around, behind and into your eyes.
Just FYI.