Denethor and Gandalf, total opposites?

The key to Denethor's failure lies far deeper in the history of men and can be traced back to the Akallabeth: "[death] became a grief to them only because coming under the shadow of Morgoth it seemed to them that they were surrounded by a great darkness, of which they were afraid; and some grew wilful and proud" (The Silmarillion). The weekness that we see in Denethor is the same weakness that brought the Numenoreans to Darkness. Seeing his sons die and the ending of the line of the Stuards of Gondor, the shaddow of death desended on him and he grew afraid. By the time Gandalf came to Minus Tirth he had already conquered Death and come back. The chief juxtaposition between the two charactors is that Denethor is "under the shaddow" and Gandalf is submerged in Light. Therefore I would say that yes the they where both instrimental in the defence of Gondor, Denethor through death, sacrifice and grief (the price paid by all involved in war), and Gandalf in the defence against the very power of the ring, which is fear.