I highly recommend the Ionic Pro. I bought mine off TV, but I have included a link for you below. I have severe asthma and this has really helped me having this in my bedroom. I have had mine for about two years. It is so quiet. I had gone through buying all those smaller machines, I think I had 4 or 5 of them all over the house. The replacement filters for those machines run around $18 - $20 each. I couldn't afford to keep replacing the filters, so I threw them all away.
With the Ionic Pro you just have the pull the blade out and wash it. When I was still working, I had a machine in my office that looked just like the Ionic Pro, but it had two filters that had to be replaced on a regular basis and it was loud. I would definitely stay away from the machines that have filters that have to be replaced.