I had always preferred to ride big two strokes but after riding my brothers crf450, I began to become less thrilled with my CR500. My search for the biggest two stroke wrapped in a modern chassis lead me to a KTM 380. The 380 (similar to the 300) was a very good transition bike that you rode like a 250 that was on steroids. Ultimately I could ride it off road and be competitive with the 450's when the trail got tight,but as soon as the trail opened up the 450's had an advantage and their weight become less of an negative. Once things got muddy or slick, their ability to hook up and stay hooked up was definitely an advantage over the 380.
I made the mistake of riding my brothers CRF 450 again once right after getting off the 380 and that was it for the 380. Purchased a 20012 YF450 and have enjoyed the suspension and power for over a year. I performed some mod's which greatly improved the off road experience (Injectioneering throttle body mod, fly wheel weight, tuner and some times G2 throttle tamer) and have zero regrets about the switch from the KTM to the Yamaha.
The only real time that the weight difference became really noticeable was going hot and deep into a turn. The 450 has more mass to slow down and bend around a corner and you have to respect and acknowledge that fact. Truth is the power that the 450 makes and how it makes it changes the way you ride after a two stroke. Some say it makes you lazy. For me it gives me options that the 380 didn't offer.
I'm sure the KTM 300 is a fantastic bike. For me, the KTM had a more narrow focus that the 450 for the off road riding I do.