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.
2003 DR 650.(1976 Montesa 250 Enduro-nice!) - - 1990 MASI TEAM-3V.- 1976 Motobecane, Super-Mirage.- Kona, HumuHumuNukuNukuApua'a. Single Speed ThRaShEr BiKe. 1968 360 Greeves challenger MXer. 1999 Triumph Trophy 1200. 2011 KTM530 EXC. 2012 KONA Hei Hei Deluxe (ongoing bike issues) -2009 KTM 200XC-W.