Hollow, simply because I own a Tormek. I grind my chisels at a 25 degree angle which works great on almost any wood species. Resharpening within a minute if needed.

I doubt that there is a great difference between hollow or flat. As long as it is sharp it will do the job. It might stand up longer if it is flat though. But I never tried it.
I am spoiled by my Tormek. If the chisel gets dull I can resharpen and hone within a minute.