There is no pre-payment, and reservations are not necessary. We made one once, just because, and never since. With the new building, they have plenty of space.
Get the app as it makes checking in and out easy (no ticket) as well as paying and applying coupons.
Cheerd.
I don’t know about the ‘reservations not necessary’ part - at peak times, I’m almost always asked whether I have a reservation (though to be honest, never really been asked for proof). I’d also say it can still be tough to find a spot, even with the newer side, at peak times. Biggest pet peave is the green/red lights are often inaccurate, I’ll go down an aisle following the green lights and often find the spot full despite the green light.
I also second the shuttle frequency as a good reason to use ParkSFO vs. Anza. Had a horrible experience with the Anza shuttle our first trip we went there, and so that was the last time we used Anza. That’s an even bigger reason not to use Anza besides the abysmal state of the lot itself - at least the area I was in.