You guys can feel free to talk about Joe Johnston as well if you like. Feel free to address your opinions on how Johnston could have done if he was kept in command at Atlanta.
OK... I think if Johnston was given more ammunition, food, and horses he could have given Sherman a good run for his money and possibly even force him back in disgrace until Sherman comes back with even more guys. Sherman's ability as a tactician don't impress me, he just had a stronger force in almost all of his ACW victories. I'd peg him just below Bragg as a tactician, but much better than Bragg, Hood, Pillow, and the other idiot generals as a general overall. If Bragg is in charge instead of Hood, south still loses and things play similarly, but with the casualty ratio being a bit more favorable to the South compared to OTL
With OTL supplies, I bet Johnston could slow Sherman down to maybe 1/3 of the march speed. He was doing a good job of harassment in OTL, and having Northern troops smack head on into prepared positions, only to retreat to a new location at the last minute. Given Sherman's uber aggressiveness, I wouldn't be surprised if most artillery pieces came too far ahead for their own good and ended up getting flanked by cavalry. the uber aggressiveness worked against Hood who can be compared to WW2 Japanese, sending men into open killing fields. Going at 1/3 of his OTL speed means Sherman is running out of supplies since he was hoping to live off the land.
In short, I think Northern casualties would be similar in a campaign as OTL, but much more casualties per mile since they aren't getting too far. Atlanta is being fought in the streets over by election time, although Lincoln still wins his reelection (McClellan was forced by his party to run on a peace platform when the war is being won by the North). The South's casualties is probably a third of that, but they are trading land for tactical advantages and they realize in 1-3 years the land will run out and their backs will be to the sea. Oh wait... a few more months Grant won the Virginia front, the Atlanta front is now a moot point.