How does a fighter mark? With his attack powers. No attack power used, no fighter mark.
Yet you need a special power beyond your ordinary marking ability to do this. Shielding swordmages have such a power as well (although fighters have more of them in Martial Power (IIRC 4 that mark until end of encounter and 4 that mark until save ends spread over all 30 levels), they only have them in Martial Power. Swordmages already have one in their base book and Arcane Power isn't out yet).
Yeah, that's fine...but I think that it's probably safe to assume that the powers in Arcane Power will probably further increase the Swordmages elemental damage potential and ability to engage multiple foes at once.
I'm not trying to say they suck or anything...quite the contrary. As I said before, they're the best of both worlds. They have the AC and marking ability of a Defender, and the area effects, elemental damage and NAD targeting ability of a Wizard.
However, because they're essentially a hybrid of those two classes, they're not quite as good as either at their specialty. A Swordmage is not as good of a Controller as a Wizard, and I don't think they're as good of a primary Defender as a Fighter.
A Fighter can tie down multiple enemies every turn through the use of powers and extra attacks granted by feats and allies, like the Warlord. A Swordmage, most of the time, is just using his Aegis to tie down one foe. Although in fact, they can't really tie people down very well. They don't have the same ability to punish moves and shifts that the Fighter does, which means while they can mitigate damage and punish those who attack allies, they have a harder time stopping movement.
So basically, if you're only going to have one Defender in the party, I would recommend Fighter. 2 or more though? Almost any combination will work, because their abilities will support each other. In the case of a Fighter and Swordmage, for example, the Fighter can tie down the bulk of the enemies and slow the enemy's movement while the Swordmage flits about and uses his Aegis and area powers to greater effect.