On my 120 gal planted, I just switched last weekend from overflow/wetdry to a canister with no overflow. I originally decided on overflow/wetdry because of a surface scum problem in past tanks.
I switched to canister because I lost tremendous amounts of CO2 during the trickling of water over the bioballs in the sump (could not get below 7.0 without running my 10 lb CO2 at a rate that would use it all up in 3 weeks!!).
Now with the canister, I have no problem with maintaining 6.5-6.6, at literally a fraction of the CO2 usage (I bet my 10 lb CO2 tank lasts 3-4 months or more). HOWEVER, THE SCUM IS BACK!
Within days the surface scum began to form. I do have an open top, and I'm sure there is some level of dust, etc settling in from the room. MY CO2 is around 30 ppm, which I have read on other forums seems to cure the surface scum problem, but not on my tank. I add Fluorish, KNO3, K2SO4 as well as CO2 at recommended doses (Tom Barr). Plants pearl and grow like crazy.
My 30 gal low tech plant tank with glass top(15w NO Fluorescence, no CO2, aquaclear 150 HOB filter, only add fluorish) has NO SCUM and never has. Plants (Anubias sp., Java Fern, Crypt sp.) grow slow, no pearling, but are doing great.
SO... if you go an open top, high tech, watch out for potential surface scum. Aquaclear makes a surface skimmer attachment for HOB or canisters. Plan B is to get one of these (cheapo at Big Al's) and give it a try.