CAUTION : INCOMING WALL OF TEXT.
Firmware upgrade and ROM are the same. v10c is technically a ROM too. Prime/ Void is a firmware upgrade too.
A custom ROM, other than the one provided by LG is a bit harder to install and may require additional stuff (Custom Recovery). But the chance of bricking during the procedure is same. It's just that LG sometimes refuses to service phones which have a custom ROM other than theirs. But you can easily revert back to stock settings and ROM.
The best ROM for performance as well as speed at the moment is, Void or Prime.
Void is a bit harder to install, but its very flexible to your desires.
Prime is easier to install, not as flexible as Void. But I prefer Void.
The procedure and all instructions along with the stuff you need to do and install is available on Xda's p500 android development section.
Link: Optimus One P500 Android Development - xda-developers
Yes, you can still install the official gingerbread update whenever it comes out ( going to take a while, I guess.)
But the awesome guys over at Xda ( I totally love em'

) have come out with a basic WORKING gingerbread already

. And yeah it works, but it's quite buggy at the moment, and I think the camera doesn't work sometimes....
Btw they have also released cyanogen mod

.
So root and install a custom ROM, a whole new world is waiting for you

.
Woah, this was long, hope it helped you
TL;DR: They're both the same, just head over to xda and install one already.