The Extreme Edition has a new transfer. I don't know about the special features. You can watch the Extreme...or EXTREME! edition in Hi-Def on the internet, which is a cool feature. I don't know if Brazil uses PAL or NTSC, but I would buy NTSC just so I don't get PAL speed-up.
The Ultimate Edition has DTS and DD tracks on it, as well as tons of special features. The transfer is all that is lacking on that edition.
The Extreme Edition has an amazing new picture master(one of the best out there in my opinion) and the DD track from the Ultimate Edition. It also has a wonderful audio commentary by James Cameron on it, his first ever.
I would recommend getting the Extreme, mainly for the much improved transfer, although the extras on the UE are excellent, but to me the movie is king.
The extended version of T2 is on both the Ultimate and Extreme Editions. There's also a super-extended version with an alternate ending thta's only on the Ultimate (though the scenes are included as extras on the Extreme).
I doubt we'll see a Terminator Trilogy in the US any time soon because 3 different studios own the DVD distrubution rights to the 3 different films (Terminator - MGM, T2 - Artisan, T3 - Warner Brothers).