The primary difference is a crank (48/31T or 46/30T vs 50/34T). The brake pivot points are re-designed, and there are a few other tweaks. One of the rear derailleur options lets you use a bigger cassette.
If it were me, I would get the 105-equipped bike, and then only change the crankset and/or rear derailleur if necessary. (The chain-line will be +2.5mm, but I think the worst-case scenerio is that you might have to get a new front derailleur.) The Sora-level bike is likely to have other compromises. Also it is much easier to upgrade piecewise if you have 11-speed 105 to begin with.
Alternatively, wait (a year?) to get a bike already equipped with it. But keep in mind it will be more expensive because it is new.