My 2 cents Brushy would be to find a LBS that has used bikes. If there are none on the floor, you can always ask if they have any. I can't tell now if you are riding, so it would be hard for me to recommend one versus another besides going by the length of commute and the road conditions that you give. And given that scenario all the types of bikes you mentioned would be fine, which of course is of no help.

If you are currently riding, what type of bike is it? What do you like or dislike about it? If you are not currently riding I would suggest the used bike first so you have a better idea of what you like or dislike about it, before stepping up to something better. I think you need a number of miles under your belt (with whatever kind of bike) in a bunch of different conditions before you can really make an informed decision. The descriptions people give about the bikes are in relation to other bikes and sometimes the experiences they have had. I think you need to have those experiences to understand what they mean. You won't understand how much of a difference something is until you have a comparison.

For example, if you have a bike do something simple like changing tires and you will have a better understanding about the differences. Ride a bike with knobby tires, and then ride one with slicks. Go to your LBS and ride a mountain bike, then ride a road bike. If you have a friend ride his 90's hybrid, then ride a CX bike.

I think your first bike should be your learning bike, and your second bike should address what you learned from the first. Another poster pointed that out to me.