I use the one that has two drums. Work fine for me.

Harbor freight took the coupon I had on my phone when I went there a few weeks ago.

Just go to their web site. They'll let you stack coupons also... at least they did for me. 20% off a single item and then like another 10% off or something like that.

I use the harbor freight beast model.

http://www.harborfreight.com/18-lb-vibratory-bowl-with-liquid-drain-hose-96923.html

Plows through a lot of brass quickly. Just fill it up with corn cob and some nufinish and the magic happens.

Coupons are good

Would this work for stone washing knife blades?

It works fine.  You will get tired of the lids and filling two things all of the time.

IF you decide you like wet tumbling spend the time to make a PVC drum for the tumbler.  Basically a four inch diameter piece of PVC as long as the two drums combined length.  Glue some 3/4" PVC rails inside to assist with tumbling.  Seal the bottom with a cap and use a pressure cap on the other end.  It is much more convenient and I think it will hold about 200 223 cases at a time.

Look in the tumbler thread for plans/descriptions of the PVC drum.

ETA:  The drum is described on pages 9-11 of the thread dryflash posted above.