There's no doubt mobil 1 will do fine in there. Dex6 just happens to be the new spec on the block that is known to have a more stringent spec than the old spec and is also back spec for previous gm transmissions.
With that said this is what mobil 1 syn atf page says.
All 2005 and earlier GM vehicles**
** Mobil DEXRON-VI ATF is recommended for 2006 and newer GM vehicles and improved performance in 2005 and earlier vehicles, wherever DEXRON is specified.
http://www.mobiloil.com/USA-English/MotorOil/Other_Products/Mobil_1_Synthetic_ATF.aspx
In the end both will work and the choice is up to you. Neither is going to harm the transmission.
With that said this is what mobil 1 syn atf page says.
All 2005 and earlier GM vehicles**
** Mobil DEXRON-VI ATF is recommended for 2006 and newer GM vehicles and improved performance in 2005 and earlier vehicles, wherever DEXRON is specified.
http://www.mobiloil.com/USA-English/MotorOil/Other_Products/Mobil_1_Synthetic_ATF.aspx
In the end both will work and the choice is up to you. Neither is going to harm the transmission.