I personally prefer Marineland Emperor series filters. The 280 is up to a 40g tank and the 400 is up to a 90g I think. I have a Emperor 400 on my 29g reef and it does great. The filters are about $4 each and I really like the media trays.
A mechanical filter removes particulates in the water column. It's not extremely necessary if your refugium has a slow enough flow where the detritus settles out.