It isn't realistic for most teams to aim for overall victories in both races, so they have to do some prioritizing.
"I guess the decision process kind of comes down to, first, which is most important to you as a team, to your sponsors, to your fans," Stewart said. "Second, and this is kind of how a team like ours will approach this, there are definitely riders, by the characteristics of the race, that favor a Grand Tour over a one-week tour."
The Tour of California covers 729 miles in eight days. The Giro d'Italia covers 2,147 miles in 21 days. Not every strong eight-day cyclist can maintain that pace over three weeks.
Conversely, "the guy leading the Giro now probably couldn't win the Tour of California," Wilson said of Nibali. "The short hills wouldn't really suit him."
That may sound strange to local cycling fans psyched to watch the Amgen riders climb the steep grades of Sonoma County in the approach to Santa Rosa.
But the cumulative elevation gain of that final stage is 7,592 feet. Compare that to Stage 19 of the Giro, which includes ascents of Passo dello Stelvio (a 5,095-foot climb), Passo Gavia (4,331 feet) and Val Martello (4,665 feet).
Most teams also take the riders' preferences into consideration. For Italians, the Giro is at least as important as the Tour de France. Native Californians might prefer to race close to home.
"The riders have different motivations," Stewart said. "Some riders are hungry for California. Probably half the team asked to come to California. Our team, I think we respect the riders' wishes. Because you know motivation comes from there. It's different if they want to be there."
That generosity has its limits, though.
As Wilson said: "If someone is better than you, you're gonna get replaced."