It your choice , parasmi rom is also better and more customized than stock rom. You can get OC from this link http://www.mediafire.com/?7shfs8938g3l3
You can find a complete info regarding parasmi rom at this thread [ROM] [Samsung GT-S5570] The ROM that has no name **Final Version** - xda-developers
ext1/ext2/ext3/ext4 all are the same formats but later ones are extended and improved versions of ext format. It is better to go with ext4 partition.