I've had a lot of experience with Cannondales. My first was a 2002 hybrid, which I love and still ride on the towpaths and old railbeds. The next year I bought a R800 WSD C-dale road bike, which was a great way to get into road biking. I'm a little over 5'4'', and the 50cm size fit perfectly. After putting about 3400 very happy miles on that bike, this spring I bought a Cannondale Six13. It's not a WSD, so I went for a little smaller bike, and put a shorter stem on. It's like driving a Porsche.
Some people say otherwise, but I really love Cannondales. And don't be afraid of aluminum. If you need to, a little gel bar tape, and Buzz-kills in the bar ends, and you'll smooth right out.
Also consider what you want to do. Is touring in the picture? Or just club rides? If you're planning on touring, you may need something else entirely.
The best thing is to go with a good shop, where you can ask anything, get any adjustment made, and be treated well.
They shouldn't mind you taking a long test ride, either. Go up and down hills, stop fast, go over bumps. Do everything you think you'll do while riding.
It doesn't get any easier, you just get faster.
Eddie Merckx