MG,
For my 2cents worth, you might find the control cable setup a little nicer, since you can feather the valve a little for speed control. The solenoid control is full on/full off. The only way you could get a little speed control with the solenoid is to use a flow control valve that will fix the speed-- you could probably use 2 valves, one to control the open and the other control the close speeds.
My vote would be for the simple--cable control manual valve.
On the valve mounting, don't hang it off the piping. Vibration or a tree branch will wipe it off-eventually. Mount it solidly with bolts and a welded plate mounting bracket
I noticed that your grapple forks are what I call "US" style, where each grapple fork is separate. I'm familiar with "Canadian" style grapples where both sides are connected together so they close as a unit. Do you need to use a flow divider to make the forks move evenly or does one close onto a log with the second one following when the first one bottoms out? I've attached a photo of a "Canadian" log grapple.
Jon.