Sam has too much of a puppy face despite his bulk. Also in my humble opinion Mr Ackles is the better actor - he just has nuances that can portray evil or anything for that matter, better. :-)

He has a history of drinking Demon blood, he chose a demon bitch over his brother in a time of crisis and he's hunted Unmercifully without a soul.

I think the character Sam would more likely go evil, but I think that Jensen would be able to play it better. Sam doesn't scream menacing...

He can look FAR more evil/demon like in my opinion! Dean just doesn't look anything like evil/demon

I think Jared Padalecki has proved to us that he is awesome at playing evil/bad roles: when he was taken over by Meg, Lucifer, soulless Sam, etc.
But we haven't ever really gotten to see Jensen play an evil/bad role so maybe he would surprise me.

something devilish about him

That's why he has played evil most often