I would say if you're reasonably sure of the fit, go the BD route. Don't let the assembly scare you off, there isn't that much to it, especially on their road bikes. You'll need to learn how to adjust the headset and derailers and maybe brakes, the park tool website has great articles that cover all of this and more. I didn't need any special tools, just a metric set of allen wrenches. Even if you have to pay for assembly and regular service, you still come out ahead. 105 group will get you very close to the very best performance wise, even if it's a bit heavier. I've never ridden the sora group, but have the MTB equivalent on my old MTB and there's no comparison to my new bike (SLX group, which would be the MTB equivalent of 105).
Or save your pennies and watch for year end sales at your LBSs but I'd rather be riding if it were me