What's at the other one?
The Holy Land of Mariejoa… at least that's the impression that is given by the fact that the King of Lyneel had to request their permission to enter the Grand Line.
And since if you start at Reverse Mountain, Raftel is the "last" island, why wouldn't it be easier to start at the "other one" point and then Raftel is the first island?
Hmmm… I don't think that you have the right mental image of Planet Oda's layout.
Think of it this way:
The Grand Line is a one way street that begins at Reverse Street and crosses another street (Red Line Avenue) before ending at Raftel Street. If you get onto it at the Red Line intersection, then you still have to drive that one block to Raftel.
Now, if you want a real shortcut, then the easiest thing to do is sail alongside the Red Line starting either in North Blue and heading south or West Blue and heading north. If Raftel is in the Grand Line before you reach the Red Line, you should be able to find it by this method. The downside to this approach is that you would first have to cross a Calm Belt to enter the Grand Line.
Anyway, the trip from the Mariejoa entrance is shorter, but I would imagine that it's a harsher trip.