On the ecological point, the difference is whether the grassland would naturally support that number of bison. Or if bison (or cows) are continually being grown, killed, grown killed at industrial scale beyond the normal bison lifecycle. With many carbon inputs (fossil-fuel based fertilizer, etc) and production costs (slaughtering, transport, etc).

Wild bison live 15-20 years.

Cattle raised for beef ~18 months.

That's beyond the natural carbon cycle an existing ruminant population.

Also horses, in my reading of the data, actually have less of a methane impact. They have a different digestive system entirely.

On the methane point, methane, yes, is a greenhouse gas that breaks down over years (~12 years). But has far worse impact on climate while its in the atmosphere.