Jurjen disapproves of people making McClellan=Clark comparisons, noting that it seems to be based mostly on no-one having anything good to say about McClellan, a rarity for American generals. He must run in different circles than I do, because there are a good many McClellan apologists and contrarians in the Civil War newsgroups. Brad Meyer comes immediately to mind.
On the other hand, I have never encountered a Westmoreland apologist. Mostly, this is because Westmoreland was sent home halfway through his war, and his replacement, Creighton Abrams, is near-universally revered among the military. The anti-war factions of the Vietnam era, on the other hand, had absolutely no use for a "butcher" like Westmoreland, and Westmoreland had no obvious military ambitions, and no political base to speak of.
McClellan would be the appropriate parallel for Clark, because he was a "victorious" military technocrat with political ambitions who was willing to seize the anti-war banner in wartime. I've argued for Winfield Scott, in that he was a retired, victorious general who became the candidate of the party which opposed his war, but you could make a case for McClellan in that he ran during wartime, while Scott ran four years into a period of relative peace, at least as far as the national government was concerned.
McClellan and Clark both shared other characteristics. For one thing, they were both military technocrats - McClellan made his pre-war reputation with studies of cavalry tactics and an evaluation of the Crimean War. Clark has been busy writing books on transformative military issues, and is a genuine, if disliked, member of the Jedi Knights. Both Clark and McClellan share "victories" which are held in low esteem by observers, while still being, nevertheless, a species of victory. Clark's Kosovo campaign seems to be a sterling example of how to do everything wrong, and still win. McClellan, on the other hand, demonstrated how one might win all of one's battles, and still lose the campaign. On the other hand, McClellan has a well-earned reputation for being risk-adverse, while Clark apparently scared his superiors and some subordinates with wild ideas; McClellan was a slow and methodical turtle, while Clark plunged into the Kosovo campaign with a minimum of resources, and had to mark time after he blew through his initial resources, while waiting for reinforcements and resupply. Clark was very lucky that his enemy had no way to bring the war against Clark's vulnerable points. In a sense, he was in a position where he had absolute initiative. McClellan would have been absolutely green with envy, I imagine.
One final difference between Clark and McClellan - the regard in which their peers and subordinates held them. McClellan's men honestly loved him, and he made his army in his image. The rumor mills are full of just how much Clark is disliked by those who served with him. In order to be a Napoleonist, one must actually be able to claim the loyalties of the military. Clark seems to be more of a Blighist.