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.
That really depends on how broadly you define 'threat'. I would say airspace violations, chemical assassination campaigns, and not-so-covert support for anti-European political factions constitute that (not to mention the inherent effect of more blatant aggression 'in the neighborhood'), but I can see the argument for saying the word should be defined more narrowly, since we're no longer in immediate danger of Russian tanks overrunning Vienna. In any case, whatever you call it, deterring Russia is necessary to ensure the cohesion of the Western bloc and the independence and territorial integrity of the nations on its eastern frontier in the long run, and both cases here undermine that goal.