Yes, the extra bit is worth it. Sora and Tiagra is pretty entry level. If you are starting and have a limited budget, get at least a Tiagra but in terms of selling the full gruppo, not very likely you'll get much back for it, if at all. If you look on Craigslist, not many people sell Sora/Tiagra components. I've even seen Sora/Tiagra level bikes stay on Craigslist for months with no buyers.
I would much rather buy a bike fully built with 105 components stock. The carbon seat post is worth having. You reduce the overall weight of the bike and you are ensured comfort when you ride over bumps. My personal belief is to stick to a bike that has components that work with the frame. Kind of odd to see an entry level road bike with a full Ultegra gruppo. But that's just me.
