There were only three routes the Army of the Potomac could realistically take:
a) The Richmond, Fredericksburg, and Potomac (RF&P) Railroad which ran from the north past Aquia Landing, a key Union supply base when it was held by the Federal troops, through Fredericksburg and directly towards Richmond. Aside from being the most direct route, it also could handle the most volume of the two railroad routes. This was the route taken by Hooker and Burnside.
b) The Orange and Alexandria Railroad. However, it handled notably less volume than the RF&P, and had another notable disadvantage; it led away from Richmond. Charlottesville was about as close as it could lead towards Richmond. The Lincoln administration liked this route because it did keep the AotP more or less between Lee and D.C. Most of the AotP generals, particularly McClellan's adherents (for example Sedgwick), disliked this route however.
c) The Peninsula-James River route.
McClellan and Sedgwick considered it the "true" line of operation to take Richmond; this route enabled the Union army to be secure on the flanks, and to be supplied by sea from the Union navy. However, the Peninsula route was inherently somewhat vulnerable to being bottle-necked, and just as importantly, McClellan's failed campaign using this route poisoned it politically in the eyes of the Republicans. Still, strictly in military terms, McClellan was quite probably correct to see this as the best route.