Well I got a manual RT and its a fine first car. Its also a J reg, but it was considerably more than 300. It had only done 40k the other year and was well looked after. Never had a problem with it, it never breaks down, always starts first time, never cuts out or any of the other problems that plague my mum and dads cars. Then again they drive like old people, and I am certain that causes cars to start buggering up. Anyway if I was you I would go for the manual, more fun, but also in my opinion, cheaper to get fixed. And if its only £300 you might need to do that more often than you would like.
Paul