Vanguards definitely have more utility when it comes to tanking. They have superior range and abilities from what I can gather and can easily focus on multiple enemies at once in all directions, near or far.
At least that's been my experience with the Vanguard so far doing hard mode flashpoints with others. I generate threat, interrupt enemies using various moves such as Riot Strike, Storm, Harpoon and Cryo Grenade, use debuffs and generally keep the enemy off balance and focused on me no matter where they are.
The Guardian can do the same of course, but the Vanguard has the advantage of being a ranged class where their abilities are good from up to 30 meters away.