With a pruned wallet possible in master I believe we should expose the option in the GUI.

The Main tab in preferences will offer a chance to toggle the node type and size.
<img width="612" alt="pruning_gui_off" src="https://cloud.githubusercontent.com/assets/216917/8766147/834fb842-2e2c-11e5-94b0-c342132243bf.png">
<img width="612" alt="pruning_gui_on" src="https://cloud.githubusercontent.com/assets/216917/8766148/8351a0c6-2e2c-11e5-841b-af02fd77ff44.png">

The prune size UI should not permit sizes less than 550.

{
    Q_OBJECT
public:
    explicit OptionsDialog(QWidget *parent, bool enableWallet);
    ~OptionsDialog();
    void setModel(OptionsModel *model);
    void setMapper();
private Q_SLOTS:
    
    void setOkButtonState(bool fState);
    void on_resetButton_clicked();
    void on_openBitcoinConfButton_clicked();
    void on_okButton_clicked();
    void on_cancelButton_clicked();
    
    void on_hideTrayIcon_stateChanged(int fState);
    void showRestartWarning(bool fPersistent = false);
    void clearStatusLabel();
    void updateProxyValidationState();
    
    void updateDefaultProxyNets();
Q_SIGNALS:
    void proxyIpChecks(QValidatedLineEdit *pUiProxyIp, int nProxyPort);
private:
    Ui::OptionsDialog *ui;
    OptionsModel *model;
    QDataWidgetMapper *mapper;
};
#endif