I looked at all of them recently when I was buying, and like you I was used to my mountain bikes. I ended up riding about all of them all of the way up to a 2.1. The 2.1 was the best - but being a new rider, I could tell very little difference in the 105 components vs. the tiagra other than aesthetics (the 105 cables are under the bar tape). I will say there is a very noticeable difference in the sora vs. tiagra. Not that the sora didn't get the job done, the tiagra shifted more quick and accurrately, and the shifters behind the brake levers are just as easy to work in the drops or on the hoods. I debated a week or two over the two. I would have liked the 105's on the 2.1, however, that was at the top of my budget and I still needed pedals and shoes. I went with the 1.5 and broke down and got some good shoes. I am glad I made that choice.
Like other have said on here plenty of times - go with the most bike you can afford. You will never regret the upgrade when you are riding.