Stone all the way. The sparger will eventually carbonate the tank, but that'll take ~2ish weeks while keeping the temp at a constant low level and IIRC you don't have cooling on your bright tank. It's barely a step above pumping the CO2 into the headspace.
I think that so long as the stone is near the bottom of the tank that the center v. off-center location is irrelevant.
If you do get a stone, be sure to install a one way check valve on it if it doesn't come standard.