When I was looking for my first road bike, I test rode 5 or 6 bikes before I found the one I liked. I was out riding on that bike for over 30 minutes. It just felt so right. The other ones had something that didn't work for me; I felt too stretched out or too cramped.
For your first road bike, no need to pay extra for carbon fiber, unless money is no object. Carbon is plenty strong and durable, but more expensive than aluminum or steel.
Fit is key; if the bike doesn't fit, you won't ride it. Frame material is further down the list. I do recommend going to wider tires for a more comfortable ride. 700c x 28 should be wide enough for comfort and still plenty fast. I can't tell the difference between 700 x 23 and 700 x 25; 28's aren't that much bigger and I weigh 165 pounds.
Talk with the shop guys. Tell them what you want out of cycling. What type of riding you want to do now. You are not too big for a road bike; don't let them sell you a mountain bike. If they do, just say thanks, but no thanks and walk away.
A nice aluminum frame with Tiagra components would be a very nice bike to begin with. If you can afford more, spring for Shimano 105 components.
But most importantly, the bike must fit you and be comfortable. That may very well mean swapping the stock saddle for something else. No big deal, most riders ditch the stock saddle. I can recommend Brooks saddles and the Selle Italia SMP line. But saddles vary as much as our posteriors so you'll just have to check them out yourself.
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