Do the pre-order bonuses offer any advantages or do they just look good?

Solution 1:

According to the developer:

The costumes can be turned on and off. When you turn on the "Knight of the Order" costume, he's still using the crossbow, but instead of the Bladestaff he'll be carrying (and swinging) the Hammer of War (it'll still show as the Bladestaff on the spellbook tiles, but don't let that fool you). You can turn the costume off at any time to go back to the standard War Mage and Bladestaff.

Same goes for the Cardboard Tube Samurai costume. If you came by our booth at PAX Prime and unlocked the CTS costume, you can turn it on to change the clothes of the War Mage and to swap out his Bladestaff with a giant cardboard tube. Yes, it's pretty awesome.

So it appears as though the suit, including the Hammer of War, is nothing more than an affixable costume that offers no additional gameplay benefit.

Solution 2:

It actually does guys. If you notice, while you are using the Hammer of War, each of your hits will make the common orcs shake a little bit, which means that the Hammer of War sometimes prevents the orcs from attacking you.