Yes, in the long term the forest will have the higher carbon accumulation, simply because in grasslands you can accumulate only so much grass biomass. Yes, grasslands also add carbon to soils.
Having said that, in the context of the interior of British Columbia in particular, a mix of grasslands and forest can alter the fire risk. Having vast areas of contiguous forests is contributing in part to the very large fires. Designing a landscape that has more of a matrix of grasslands and forests may help reduce fire risks.
These questions have arisen out of the context of the fires in British Columbia in the last two years. Much research will be directed in the coming years on strategy toward reducing forest fire risks. For example, the Pacific Institute for Climate Solutions has an active program on forest carbon management and opportunities in British Columbia.