When I purchased my home I installed a space gaurd model 2200 on my unit and I am very pleased with it.
Something I really like about the spacegaurd is that the filters are easy to find and if you take your time easy to install.
I see a lot of dealer only filters out there (bryant,trane,carrier) they all have them and claim to be the biggest and the best but the truth is the aprilaire works just fine and if you ever switch service companies the new company will have the filter in stock.
Good luck
post some before and after pics those are always a treat