Command Point One, Attackers Spawn
The Grenades here are an early advantage to the defending team, who can use them quite effectivly to spam and harass the attacking team as they leave their spawn. They aren't easy to defend once the attackers have broken through, however. If you can keep control of them however, it will give your Engineer's time to set up their Sentry Guns to defend the Command Point, so it is worth delaying the enemy as much as possible here to allow them the chance to do so properly.
The chokepoints, shown in the picture above by the Blue Arrows, lend themselves to protection by Soldiers, Demomen, and HWGuys. They can also be a good point for Pyro's to check for Spies - narrow corridors don't give cloaked Spies much room to avoid the flames.
Conversly, for the attackers, it's very advantageous to carry momentum and break through quickly, to deny the enemy Engineer's the chance to get set up. If that can be done, the Command Point is often captured soon after.
In terms of routes to the cap, the one on the right is preferable in most circumstances, as the building it leads to houses a ladder to a roof, from which a Conc or Grenade jump can help to push the flag right to the Command Point, if not capture itself. If you get bogged down, and the Extra route opens however, it can provide a similar, but slightly more open and risky, platform for a jump as well - as well as a good vantage point for a sniper to take down defenses around the Command Point.