There was a public meeting on June 6th for the Long Bridge Study (it was not on my radar and I did not go). There is some presentation material online, but it's hard to interpret without the presentation itself. This material has some photos and a schedule (final report in 2013) and here are the various alternatives presented without commentary.
There are 10 alternatives, including no-build. Only alternative 4 through 8 include a bicycle path. Each features a 12 foot wide bike ped path with - if the orange dots represent connections - connect at Long Bridge Park, the Mt. Vernon Trail, Ohio Drive on both sides of East Potomac Park, the Anacostia Riverwalk Trail and in SW DC.
Alternative 4 starts in Long Bridge Park where a ramp passes under the railroad lines and goes up to the downstream side of the bridge. The path would be 12 feet wide along the side of the bridge and then ramp up above the rail lines to 12th street on the DC side.
In Alternative 5, the path would start beneath the railroad tracks in a tunnel it would share with the streetcar. Then it would cross the river and the channel on the downstream side before passing above the railroad tracks at street level all the way to what appears to be 9th Street (?).
Alternative 6 for cyclists would be the same as 5, except that the bike path would be next to a lane for auto traffic.
Alternative 7 is the same as 6, except that the tunnel and bridge are wider to accomodate another streetcar track and auto lane.
Finally, there's Alternative 8 which moves the trail to the upstream side.
The advantages of being downstream are a better view (since there are no bridges nearby to the south) and more sunlight - which will help with snow removal. On the upstream, users would not have to pass under the railroad. Otherwise I'm not sure of any advantages or disadvantages. It would really depend on what the connections look like.
Still, any of these options would create a great new connection between Arlington, DC and the trails along the river.
[Also, what's up with Google calling this "Box Car Willie Park"? Isn't it Hancock Park.]