Which sleeper should we book

We are a family of 6 people travelling from Beijing to Xian on 17th Nov, 2012. Our family consist of 2 adults + 3 senion citizens + 1 child of 11 year old. Please recommend whether we should book hard sleeper or soft sleeper and how many days before we need to book our train tickets. We are thinking of booking Z19 train. Is this a good train. We are staying at Days Inn Joiest Beijing and in xian staying in Grand Mercure Renmin Square. Please tell us how far is our hotel from the railways station. Please tell us whether we have to book from Beijing East / West/ North or South railway station.