I got the Xoxide X300 and my friend got the AMS gTower. I'll divide the comparisions up so it's easier to read.
Looks - Overall, the AMS looks better. The front bezel is cooler, and the side window allows for a larger, unobstructed view of the internal components. I'm not sure if the standard gTower comes with it, but my friend's has a top window too. The inside is pretty much the same for both. The X300 does have a cool power button though.
Cooling - The X300 wins hands down. The gTower gives the illusion that there are holes for the front fan, but you'll notice otherwise once you remove the front bezel. The front bezel is actually less than an inch away from the fan, and the air holes are too low for a direct flow of air to the front fan. You can even see it in the side view of the case on the website. With the X300, even though there is no front holes for air, there is at least an big opening in the bottom of the bezel for air. Also, the bezel leaves much more room for air to move in. Plus, an 12mm fan is much more useful as an intake rather than exhaust, for the following reason: postive pressure slows down dust buildup, and while the back fan usually have an unobstructed path allowing it to work at full capacity, the front has to fight for air thus lowering the cfms moved. The X300 also comes a side fan, not at the best location but still an extra fan. The side fan is pretty quiet for the amount of air it moves. In my case, I just have 3 case fans, 2 slow and silent NMBs (one intake and only one exhaust) and the stock side fan. My case is pretty quiet even when dirty and the case temps never go above 30 Cs.
Construction - They are pretty much made the same, except for the bar at top of the case, near the psu. That bar makes it hard to move the psu wires around and is pretty much useless. The chasis for both is pretty sturdy. The build quality for the AMS is somewhat less than the X300, since one of buttons (they are buttons, not screws) on the side of the AMS fell off in the first week.
I wouldn't get that case at X-Oxide though, the same case for $46 at
bestbyte. I bought the X-Oxide case during X-hour (supposedly a big sale) for $49. :|