Got into a long argument with a friend over this today. Long story short, I argued the latter because Turkey is a huge power player in the Middle East and Caucasus and practically the only thing keeping the Russians out of the Mediterranean, and all of the resulting post-British states would likely remain in the Western fold; he argued the former because it undermines NATO's second-strongest military and power projection capable power, likely seriously damages the European economy, and sets a precedent of successful secessionism in Western Europe, and Turkey already isn't a reliable partner anyways.
We figured both of us were somewhat biased by our academic specialties (I'm a Eurasia defense specialist, he a German-speaking political economist), so I would be interested to hear other opinions (or other options for similar discussions).
Russia is not a threat to Europe, with the exception of Ukraine.