Truthfully the game is a port, but it has a lot of stuff that wasn't in the Dreamcast version, and also it has the stuff that was downloadable for the DC version already in the GCN version. It's bascially like i said the Directors Cut, it has new characters. If you played it already on the Dreamcast then I don't know if you'll want to buy it again, unless you really liked it (like me

) I personally can't tell any differences graphically, as I always thought the DC version looked awesome.
As for it being exclusive, the DC isn't around anymore and Sega aborted the PS2 version in favor of the GCN, so in theory it is exclusive, as this version of Skies of Arcadia is not available anywhere but on the GCN.