OK, I just went through this.... will try to save you some time while biasing you with my opinion.....
I wanted a air filter. I am always paranoid with wood smoke/ash (even in tiny amounts), my wife has bad allergies, we have a cat, and a little one on the way. So I wanted a air purifier, even if it is just a placebo effect.
1) those ionic filters produce dangerous amounts of ozone, and dont filter worth poop. Skip them!
2) the onyl filter worth anything is a hepa filter
3) filters are expensive to replace, and you know you wont do it on time. Some purifiers have lifetime filters, you can just vaccuume them out (guess you should do it outside).
4) look at the area you want to keep clean. size the purifier correctly! I bought a large purifier for a pretty small area, this way I can run it on low. If you get a purifier that can clean X sqft and put it in a X sqft room, then it needs to run full tilt!A big purifier running on low will probably be quieter than a small one on full power.
I ended up getting a honeywell 50250
http://www.amazon.com/gp/product/B00007E7RY/ref=oss_product
There is a really detailed review, with lots of tech specs on amazon, her eis the link to it:
http://www.amazon.com/review/RPK089GRGVMP9/ref=cm_cr_pr_viewpnt#RPK089GRGVMP9
thats where I bought it. I am pretty happy with it. There isnt any dust in beams of sunlight anymore. I also looked at the black pre-filter, after 3 weeks its pretty grey. So it clearly pulled some nasty stuff out of the air, every bit of dust that goes in ther eis great, because it is better than going into my lungs (I am saving them for the asbestos in the cube farm at work).
Here is how much power it consumes, I measured with my kill-a-watt.
high - 175 watts
medium - 121 watts
low - 83 watts (24/7 at my electricity rates, this is about $9/month)
All air purifiers will make air noise, on low this one is pretty quiet. Some people report that the prefilter smells for a little while after you open the package. Mine didnt, but open it in the garage if you are concerned. I would deffinitely reccomend it!
Hope that helps,
Rick