@scribe I'd highly recommend using DC as it will allow you to use bigger batteries. See the models that are billed as "solar generators" now. (They're generator-shaped 🙄)
If you use USB then you're somewhat limited to 5V 2.4amp output from the panel. If you want more from the panel you need to use more USB ports at once which means you might be "discarding" some useful solar juice if you're not using all the ports at once. DC is pure power from the panel.