I am a proud owner of both a 450 '05 and a 300 '10.
I have ridden only nasty enduros with the 450 up until recently, it has now been dual sported. The 450 has been incredibly reliable.
The 300 feels a lot lighter when handled standing still so to say, loading it on the hauler, lifting it out of mud etc. when driving I must say the difference is not that notable. However the 450 has a bit more weight on the front wheel.
The 450 has a slight tendency to stall when lugged down low, the 300 just keeps going. So imho the 300 feels both torquier and snappier(when coming on the pipe.) I ran a really tight enduro with the 300, with many steep climbs and was really impressed with its performance. No problem getting the power to the ground, it ran like freight train. On some spots the wet mud gave way and deep, fotpeg catching ruts was appearing on many spots. Just second gear, and sit back a bit and roll the throttle hard and the bike wheelied through it all we me kind of just clinging to it, it kind of knew by itself how to get through
The suspension on the 300 works very good in mx-like conditions with whoops and jumps, but it plain sucks in the rock gardens where I dwell most of the time. I think my springs are too light, looking into getting 0.5 kg springs.. (weighing in att 115 kg dressed)
The 450 has now a 40 teeth rear sprocket, still possible to take into the woods on the lower gears, and works wonders on the long stretches of dirt roads.
So, get the 300, but keep the 450.