It's very simple.
When using raw flash access, they use YAFFS (they use it on the Nexus S too!!)
When using flash that has a disk emulation layer, like the Samsung's oneNand/mNand, then they use EXT4, because YAFFS doesn't work on that, and wear leveling is already implemented at the emulation layer.
That's what the Nexus S also uses, and the Galaxy S uses (the SGS only uses that in fact, the Nexus S has a YAFFS part and a EXT4 part)
They're both just fine and using EXT4 on top isnt especially always faster or slower than YAFFS, it depends on the chip implementation too.
can only edit post for 1min and phoronixwent down for 2 min when i edited lol..
Anyway http://project-voodoo.org/ to make ur SGS use EXT4, its not a bad hardware memory issue but a software bug. Also XXJPU firmware fixes it (but its a beta)