The sdcard image does not change the bootrom on the device but the l4t packages or so-called “sdkmanager” will update that.
It means if you only use the sdcard image and never try sdkmanager, then the bootrom on your device will always keep at the oldest release. For example, if you buy jetson nano in the period of our rel-32.1, and you never flashed the whole board with sdkmanager then, then no matter your card is using rel-32.3/32.4.4/32.5 software, your bootrom on device is always the 32.1. This may cause problem if one day there is incompatible issue between bootrom and image on sd.
Thus, I think sdkmanager is the better option.
Update: after jetpack 4.5, fresh sdcard will update the bootrom/bootloader on device QSPI too. Thus, not limited to sdkmanager anymore.