A standard 45ft sailboat with a good diesel engine should get you at least very close to 8kts under power. As an over-generalization, the motorsailer is more likely to get you a helm station protected from the weather. But there are also pilot house sailboats that can offer that.
I think it depends on why you are interested in a sailboat at all. If it is to avoid the diesel expense on a long trip, I would strongly lean toward an efficient sailboat with a motor backup. A motorsailer is sort of the other way around.