both work the same, but I found that the lg support tool seems to work better with official latest roms. The other need if you want to flash a certain kdz, not available via the support tool.
it will wipe all. also sd.
best practice, only if on custom recovery wipe system data, cache and dalvik.
If not on custom, afterwards do a factory reset. Even if not on custom recovery. This so you have a clean stock situation.
Sent from my LG-D802 using xda app-developers app