I think it is rather easy to make the decision depending on what you want. Kershaw has the greater tools and is undoubtedly the greater pitcher over the time they have been in the ML. But at this point if it is the seventh game of the World Series I would take Bumgarner. Don't forget that pitchers like Lefty Gomez, Mickey Lolich, and Lou Burdette were outstanding in various World Series of the past but no one would take them over Kershaw just because of that. Also I tend to downgrade Bumgarner's performance slightly because KC was not exactly the 1927 Yankees. Actually KC was a pretty pathetic overall offensive team that had tremendous relief pitching in these days of 5 and 6 inning starters, the best defense in baseball and very good team speed in every aspect. Their hitting was just barely good enough to keep them in games. So I hesitate to rate Bumgarner's performance as the best ever in a WS. After all, I saw Koufax in the 1963 & 1965 Series and Gibson in 1967 & 1968.
Ironic you mention Lolich and them come around to Gibby on the 1968 series. Lolich was 3-0 all complete games with 1.67 ERA including Game 7 on 2 days rest over Gibson. Without going to far out on a limb I would say the 1968 Cards were a far sight better than the 2014 Royals.
Never understand why this effort is totally lost when talking about the World Series. It was a good as any.
But make no mistake. If you had a choice between Gibson and Lolich...gimme Gibby. He was nasty and his stuff was wicked too.