You would really have to ride each,the 350 involves higher rpm whether its on a trail or dirt road,a 450 that has been made to run mild so as to be street legal from the factory should be pretty smooth. Maybe your only looking at the dirt only models. When resale time comes,the dual-purpose bikes are WAY easier to sell,and they dont weigh much more then the dirt only bikes. As far as I know KTM doesnt make a street legal 450 so thats it for that idea.
Really.........for strictly dirt I like my 300 lots better then any 4 stroke,personal issue.
Some bikes around at times