Like you I had a quad (YFZ450) and went to a dirt bike.
Go with the lightest bike possible. A 250F would be a good bike for you. You will probably outgrow the motor quickly, but you should build a foundation and then move up. You will have a lot more fun and get confident in your skills quicker.
I would recommend a motocross bike. Their light, have great suspensions, and generally have the tricker stuff for their years.
I have had a YZ426 (similar to a WR426) for the last year and a half. It's a heavy beast. I would go down a dozen times a ride. I pretty much stopped riding and was really discouraged.
I just purchased a YZ250 and man am I a way better rider. I've been out twice with it and haven't fallen 1 time. And I'm riding twice as fast.
Hope this helps you.