The M4A3 deals more consistent damage over a larger area, that is out of question. So it does more overall damage.
OHK is overrated, especially if you compare HE Sherman and T34/P4H shells. I'd rather have shell that make infantry retreat consistently, then a shell that misses a lot, but if it connects will wipe more models. Only P4J with its insane scatter scaling with vet may be on par with HE Sherman in this regard.
M10 is a joke, since it is only useful as a throw away tank to trade for a super heavy in a risky dive. The Jackson rush vs P4J is suicide, since you simply gonna lose the map.
what Vipper actually meant, that from all generalists mediums 76mm has really good pen (HVAP is as good as comet's 77mm), and if you vet it, the penetration is really close to what tank destroyers have. Basic AP shell is meh for infantry, but is still enough to kill few models here and there, especially with pintle installed with potential to fend of a P4.
Swapping "problem" is way overblown on this forum. The delay is as long as reload itself and it is only an issue if you do it in armor combat, a.k.a "get caught with your pants down".