In 1861, you could have made a case either way.
For the CSA, you could have said that they had the best officers because most of the officer corps of the Army came from the South. You could also have said that it would be easier for the South to win because they would only have to defend long enough for the North to get tired of the war and give up. Finally, you could have argued that Britain would recognize or even help the Confederates because of their dependence on Southern cotton.
For the USA, you would have pointed to their larger population. This was particularly true since so many in the South were slaves. You would also look at the huge advantage the North had in manufacturing. Finally, you would say that the North had much more of a navy and would be able to prevent the South from getting supplies from Europe.