If you were to try to trim the aircraft for straight and level without the engine you might find it only works for a short time!!
It seems to me that your best chance is to keep the aircraft under control, select best glide speed and aim for the trees with the wispiest looking tops. With this in mind I suppose a pine forest would be better than a forest of oak trees. If you're lucky you might get a slow enough deceleration to let the a/c settle down onto the tree tops. If you're unlucky it will catch a wheel and catapult you downwards base over apex. Bit like ditching in the sea really.
If the trees are well spaced, aim for the gap between them and hope the wings shear off and the cockpit stops before it hits the tree that fills the gap 40 yards on.. A friend's Cessna was written off when it overran into a hedge (crap pilot), but the cockpit went neatly between two bushes and the culprit walked away without a scratch.
All conjecture, really. If you're in the situation just do what seems best in the 20 secs you have available from 150 ft.
------------------
"Take-off is optional, Landing is mandatory"