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.

bool getProxySettings(QNetworkProxy& proxy) const;
    bool getCoinControlFeatures() const { return fCoinControlFeatures; }
    const QString& getOverriddenByCommandLine() { return strOverriddenByCommandLine; }
    
    void setRestartRequired(bool fRequired);
    bool isRestartRequired() const;
    interfaces::Node& node() const { return m_node; }
private:
    interfaces::Node& m_node;
    
    bool fHideTrayIcon;
    bool fMinimizeToTray;
    bool fMinimizeOnClose;
    QString language;
    int nDisplayUnit;
    QString strThirdPartyTxUrls;
    bool fCoinControlFeatures;
    
    QString strOverriddenByCommandLine;
    
    void addOverriddenOption(const std::string &option);
    
    void checkAndMigrate();
Q_SIGNALS:
    void displayUnitChanged(int unit);
    void coinControlFeaturesChanged(bool);
    void hideTrayIconChanged(bool);
};
#endif