If by sump you mean wet/dry filters (trickle) then it may not be that bad of an idea. Here's an article I was referred to:
Concerned about CO2 loss in a trickle filter?
Apparently, much of the CO2 bleed off comes from the overflow part of the filter. The trickling over the bio media isn't a problem if the area is covered or contained. However, I just got finished setting up my 100 gallon planted and I never got my CO2 above 9-10 ppm. I have a ProClear 175 wet/dry and the bio media area is covered, but by no means "airtight." I put some coarse filter media in my overflow to stop the "splash" effect from the water cascading down and lowered the overflow in the tank. This helped reduce the water agitation in the overflow. I also added an aquasilencer (Stockman) to the standpipe part of the overflow to reduce all the gurgling and splashing of the drain. This actually got my CO2 from 7 ppm to 10 ppm.
Unfortunately, I never found or developed a means to seal or cover the overflow or make the bio media area airtight. I think this is why I could not attain higher CO2 levels but I may be wrong. I run a pressurized CO2 system. I increased my bubble rate from 2 per sec to roughly 4-5 per sec with no increase in CO2 levels. I'm new to all of this but I assume that meant the agitation somewhere just wouldn't allow my tank to hold more CO2. Higher bubble rates might have yielded me a higher CO2 level but I was quick to surrender.
I currently run two canisters, an Eheim 2217 and a Magnum 350 Pro (no bio-wheel) and now I easily attained a CO2 level of 24 ppm. The ProClear is sitting on the sidelines for now. When I get some time to make something to seal the overflow I'll consider trying out the wet/dry again. It made for a nice place to put my CO2 reactor, grounding probe and CO2 controller probe. Luckily I have in-line heaters so I don't have to hide those. Currently I have no fish as I haven't squared away all my water parameters and dosing regiments. So I'm not worried about underfilteration. I think I may be able to house a small stock with the current filtration. If not I'll have to trade of sell the ProClear.
I'd say right off the bat go canister. Although, I wouldn't discourage you from trying a sump as I've read some articles and posts that say it's possible. I can only relate to my experience, which isn't much. I hope this helps. It's been two weeks and my plants are really starting to take off.
Jesse