The northern states would probably be better off, on paper at least, all other things being equal.
The southern states themselves, and oh yeah, all the black people inside of them? Not so much.
Also, there would have been multiple civil wars. Do you really think the USA and CSA would have come to a nice, happy agreement on who gets to have California, Kansas, the Interior West, etc?
And, the South had plans of aggressively taking Spanish and French territories in the Caribbean that would have likely resulted in British intervention. Remember, they were rooting for the Confederates partly because they were banking on the Southern states being too fragmented to form a union, and then Britain's got colonies again. So, proto-World War opportunities there, plus an outside chance of Spain or France (especially France) grabbing some pieces of land in the South again if the wars went badly. Not good for the USA.
Finally, the slave states that remained in the Union would have had widespread rioting and private militias backed by former slaveholders, who saw slavery banned in 1863. They would have wanted to join the Confederacy. Basically, guerrilla warfare in Baltimore and St. Louis.
Still sound like a sweet deal for the Union?