I'd only be concerned about how fast you would outgrow the 250. It might be a waste to buy and then buy something else next year, and not be able to recoup what you paid for it.
The 250's are great to learn on, but if you already know how to ride, you might be better off going to a 600, and then be able to roll with that for a few years.
I really hate this argument.
A 600 is not a beginners bike.
If you are learning to ride, do not fall in to this trap.
If you by a 250 Ninja, for less then 1500...
You most likely with be able to sell it for the same amount in a month, or two years.
Sure the turn over rate on the bike is fast, but that means people will be looking to buy it also.
Fact is, you will not be able to buy a 600 for the same price (unless it is a pile).
Once you drop the bike a couple of times, the 600s value will drop... the 250, it is expected...
My first bike was a 250.
Also, my current bike is also a 250...