| Both the KTM and WR are great bikes, but if you are new to off-roading then I would advise against them. They are both full on dirt bikes (the equivalent of buying a ZX12 as your first road bike!) and are extreme to say the least.
You can be certain that you will fall off over and over again...so I would suggest a bike you don't mind bending a bit and learning your trade on first.
I was in the same situation a couple of years ago and went for a 1999 DR350 enduro (See avatar) after much thought and advice. These are now superceded by the DRZ400 which would be an ideal choice for a bit of everything (green laning, enduro, the odd trip onto a dirt track).
If you want something just for Motox then thats a different story.
As a 'beginner' you could be getting yourself into more trouble than fun if you opted for something too focused. A friend of mine who has been riding green-lanes/enduro for years has just changed from an old Honda CR250 2 stroke to a KTM450 4 stroke...and at the moment he is still trying to get used to it...and if he was honest with himself, he would probably swap it for something a bit less 'violent' in an instant!
You said it yourself, that you had a hoot on a 250...so perhaps (obviously depending on your budget) a Honda CRF250, DR 250/400 or something like that.
And as a matter of personal choice...I would always go down the route of a 4 stroke.  | 
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