If that's the rifle you want, 20" should be fine.
But that's circular logic - you DON'T want the 20", unless you're in a vacuum (no wind), for LONG range, which is what these are for. If they're not for LONG range, then there's no reason not to just grab a T/C Icon (or Tikka T3, or...) .308 with a thin barrel, to have a light rifle - it's guaranteed sub-MOA, and it will serve very very well to 300 or more yards. The ONLY reason to get a HEAVY rifle like that (even the 20" bull 'sniper / tactical' rifles are on the heavy side), is for LONG range, and at long range, you need all the BC & velocity you can get to buck the wind. My opinion, anyways. Now in a superior chambering, like 6.5-284, I could see going down *maybe* to 24" or even 22 as the shortest.
These 20" so-called "sniper" rifles are for police / swat type uses, where a typical shot would be 100-150 yards, and a very long shot is 300. That's fine in an urban setting, where you get out of the SWAT truck, and take the elevator to the roof - no need for a light rifle -the heavy barrel is not needed, but it doesn't hurt anything either. If your gun is gonna be heavy, my opinion is that it needs to be able to reach out there a long ways, or you're not getting anything for the tradeoff of extra weight.
Boba Fett, that article you linked to talks about barrel length affecting inherent / mechanical accuracy, which as we know, it doesn't - that's an article for newbs that think barrel length might affect actual mechanical accuracy. It most certainly DOES affect the more important consideration of PRACTICAL accuracy at longer ranges, because it minimizes the effect of your wind drift estimation error, and minimizes the effect of your ranging estimation error.
If the goal is just to look cool and be accurate to 200 yards, let's say, and haul it no further than from the pickup to the bench at the range, then by all means grab a 20" .308. But if the goal is to get the best tool for a specific job, then.....
And don't get me wrong - I am the *king* of liking short short handy rifles in the 16-18" range for walkin'-round and/or self-defense rifles - that's where short barrels have their place.