What has happened is that there has been a change in how the seasons are being split and when the portions are airing. For seasons 1-6 you had the full seasons separated by the Christmas specials. Even season 6, which was split into sections as season 7 is, still aired all 13 episodes between Christmas Carol and The Doctor, the Widow, and the Wardrobe.
However this season was split differently with the first portion of season 7 being aired before the christmas special, the Snowmen, and the rest being aired after it. This was done for a number of possiblereasons. For one, this seasons episodes have been larger production wisewith each having more location shoots and bigger sets which likely led to higher production costs, the split may have allowed time for the production crew and the actors to recover. It may have also functioned asa way of distancing the introduction of Clara and her tenure as companion from the departure of the Ponds. It is also possible that the split has something to do with the series gearing up for the 50th anniversary in 2013.
So the second half of the season and then the entire season will be released following the airing of the remaining episodes of season 7. As of now, there does not seem to be an airing date for thenext regularepisode of the rest of the season following The Snowmen. It is likely going to be sometime in April or May. For now, you could purchase the first part and then purchase the second when it finally comes out or you may want to wait as the full seasons generally have all the special features as the half seasons as well as others. If you want to watch the episodes prior to that, maybe itunes or amazon downloads will suffice for now.
I hope that helps.