Neither is nuclear. And solar PV is vastly easier to scale up. The peak rate of fission power plant nameplate capacity construction was ~1985 and at that peak rate it would take something like 60 years to build what is needed globally.
You can't "scale up" the frequency and regularity of sunny days in a year, the weather is beyond your control. In contrast, nuclear fission is one of the most predictable physical processes known to science; you just need to build the plants.
