Biowheels cause too much surface disturbance/allows too much CO2 to escape for a planted tank. Regular HOB filters work fine as long as you have the water level right to the lip of the outlet so it doesn't waterfall. But for a 40+g tank, I agree with the above poster. Canister will be the way to go.
I have had all three, regular HOB's, Biowheel HOB's and cannisters. I've been seeing a lot of posts about Biowheels causing CO2 loss lately and decided to take a logical look at all 3. IMO, all HOB's cause CO2 loss. I don't think it's an issue of Biowheels. The Biowheel itself doesn't hold a lot of H2O as it turns, so I don't think there's a lot of CO2 being exposed to the atmosphere. It's basically just "dipping" the Biowheel media into the return H2O to keep the bacteria damp. If the concern is the amount of agitation caused by the Biowheel on the tank's H2O surface, I don't see that either. If anything, the Biowheel usually causes a "softer" return of H2O to the tank, so the concept of increased agitation of the tank's H2O surface may be more or less a myth. In any event, if you run low enough H2O level you'll always have a lot of surface agitation, regardless of filter type.
The highest H2O/air contact in an HOB is due to the turbulance in the chamber between the pump outlet and the filter media, not at the Biowheel. On almost every HOB design I've looked at, this area is vented or open to the atmosphere. On filters that use foam for biological filtering there's even more H2O surface area exposed to the air to off gas the CO2.
Just my $.02
All that aside, I whole heartedly recommend cannisters. The maintenance is incredibly reduced, and the tank is crystal clear.