As above, if you know the size you want, can set it up for yourself and you're not worried about having a local shop you can go back to if you've any issues, then buying online can save you significant money and gives you more options. Canyon and Rose are especially tempting.
If you will need assistance to figure out sizing, etc then a local shop is the safer bet.
I think if you use significant LBS time for advice or fitting while figuring out what to get you should be willing to pay a small premium for the bike. The LBS does have additional costs so if you take advantage of the additional service they provide I don't think it's right to then go elsewhere for the same product.