1) Which is more popular?
.40 S&W
2) Which is more expensive?
10mm Auto
3) Which one is easier to buy during this time?
Depends on where you live and what's trendy there.
4) I know everyone has different opinion, which guns is the best in these two calibers? I live in Kalifornia.
Depends on what role you have in mind. For general purposes, .40 S&W is better. For special purposes, such as wilderness defense, 10mm Auto has the edge.
As my username suggests, I'm a fan of .40 S&W. In my honest opinion, 10mm Auto is overrated. It's expensive, most factory loaded ammo is loaded to FBI Specs, (meaning it's literally just .40 S&W in a longer case) it's "awesome power" is only marginally higher than .357 Magnum, (the only .41 Magnum loads it approaches are the watered down loads in Remington's reloading manual) and for a cartridge which is supposedly making a comeback, nobody is making any new firearms chambered specifically for it, leaving you with an amount of options which can literally be counted on both hands.
So ultimately, it's a special purpose cartridge with a cult following of diehard fans.
As near as I can figure, reports of its comeback have been greatly exaggerated, and in my honest opinion were nothing more than a poorly executed attempt at viral marketing by diehard fans within the industry of firearms journalism who attempted to use the declining popularity of .40 S&W as a springboard to promote the 10mm Auto which wasn't really successful at all. Fans will angrily disagree, but the fact of the matter is that nobody is actually making any new firearms designed specifically for the cartridge, just half-hearted rechamberings of .45 ACP pistols which have been around for many years.
If you like 10mm Auto and want a 10mm Pistol, then by all means scratch that itch by getting one, but don't buy into all the hype and rush out to buy a 10mm Auto based on unsubstantiated reports of a comeback which has supposedly been happening since 2016 yet hasn't really gone anywhere unless you count a handful of companies offering a 1911 chambered in 10mm, Glock making a new variation of the Glock 20, or Springfield Armory rechambering the XD45 for 10mm.