IIRC, ER6i are marketed to ipod users with a 3db(?) bass boost over ER6 to compensate poor bass-response out of ipod headphone-out. But I've also heard on this board that from pure sound quality point of view, ER6 > ER6i.
Between SF3 and ER6/i, I think it is really up to what you prefer. SF3 and ER6/i has very different sound signature; ER6/i emphasizes treble, and SF3 is more balanced throughout entire sound freq spectrum. So if you like brighter sound and a bit more clarity and detail, go for ER6/i. Some say they sound harsh. Otherwise get SF3.
There is also UM1(~$100), IM716(~$70) and the recent V-Moda(~$100) that fit into your price range, they all have a lot of supporters. Do yourself a favor and use the search function, you will find plenty of reviews/opinions on them and all the other models you have mentioned.