Simple solution: Next time you are out, spend 20 min and test it.
Find a reasonable location the 40ft cable will reach and hook it up with an amp meter in the circuit (preferably right at the battery).
Write down the voltage and amps.
Then leaving the panel unmoved, connect with the 70ft cable and check the voltage and amps.
Try to do it on a clear day and fairly quickly. Otherwise you risk variations in the solar input (like clouds casting a shadow) messing up your estimates.
But in general terms, if you can reach the preferred location with the 40ft cable, use the shorter cable. If you can eliminate shadows, it's almost always worth it because a bit of shade on a panel can pretty much kill all output from the panel.
