So, in case anyone is still on the fence about it, I tried out SOL, which was a big deciding factor for me, and it felt meh. It was very arcade-like, forcing the user to hit a button to spin without changing heading, the voice acting wasn't that great, the story was a bit average, and even with settings all the way up and MSAA turned on, the graphics were sub-par and there were STILL jagged edges on everything.
On top of all of that, the control scheme just felt weird. Scrolling the mouse wheel adjusts speed. Okay, makes sense, it's like the throttle on a joystick. Clicking fires the "machine gun" type weapon. Makes sense. If you have a target, center click is to match their speed. Okay, makes sense since the mouse wheel controls speed. Right-click to spin without changing heading. Not the best place to bind that action. Then, Space is to select a target you're aiming towards (doesn't always work), 2 is to cycle through targets, and if you have a target, you hold W to set just how locked on your missiles are and then release to fire them.
It just didn't add up to an experience that was all that great.
Oh, but All Zombies Must Die is really fun. It's like Burn Zombies Burn, except it gives PC users the coveted co-op modes, and attempts to add more humor and structure to it.