The real question to ask is how often do you want to clean your system? Because air filters will delay dust accumulation on your rads, fans and inside the case but they will certainly not prevent it.

And just for the record, "positive pressure" doesn't prevent dust from entering the case!

So basically with filters you will have to clean up less often, maybe once instead of twice a year without filters. However you'll get less performance from your radiators or you'll need to rev up your fans to compensate for the loss of airflow every day of the year.

As far as I am concerned I'd rather clean my rig a little more often and enjoy the better performance and silence everyday