Those bikes have loyal fans, for good reason.
The powervalve is adjustable on the 300, so you can tune when it opens. Not sure if the 380 overbore keeps or does away with the powervalve.
Very reliable. Good quality parts. Makes a good choice for a dual sport 2 stroke.
Like any bike, need to be set up for your preferences and the type of riding.
KTM's tend to feel 'big' if you are not used to them. Once you adjust to the feel, they handle well.
Never ridden a 380, but the 300 has plenty of power - more down low and mid-range than any 250 I've ever ridden. Peak HP is probably similar to a 250 MX bike. With good traction, the 300 will usually pull a couple of bike lengths on a 250 MX bike on a run through the gears.
You can lug the bike way down low and ride it lazy, a gear or two high, when you get tired. Most 250's wont' tolerate that.
Stock seat is hard and the suspension benefits from revalving, but there's a good chance those have been taken care of by the seller.