Upgrading from Sora can be quite pricey and needs to be considered carefully. My wife upgraded once parts of the Sora system needed changing - rear mech, cassette, chain and front ring. The cost was therefore ameliorated by needing some parts. She decided to make a complete jump and put Campag Veloce on her bike as she was disenchanted with the cheapo Alex wheels and was planning a wheel change as well. (Campag needs a different rear wheel hub).
To be honest, Sora and Tiagra both work pretty well and for most people satisfy most needs. I would suggest that you stick with what you have, read and ask around, then make your upgrade (if you need it) based upon a well researched plan. It may take you down the Campag route.