It's not at all, I own a 300 and have ridden a well set up 250r. They're super fun bikes, the main difference is FI and midrange. Carbs aren't the end of the world at all and the midrange difference isn't too big as the 250r will still get up and go, though not like a 600+, at the top end.
I say go for the 250r you can afford rather than the 300 you can't
__________________________________________________
I see you over there seeing me, do you see the me I think you see?