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.

QAbstractListModel(parent), m_node(node)
{
    Init(resetSettings);
}
void OptionsModel::addOverriddenOption(const std::string &option)
{
    strOverriddenByCommandLine += QString::fromStdString(option) + "=" + QString::fromStdString(gArgs.GetArg(option, "")) + " ";
}
void OptionsModel::Init(bool resetSettings)
{
    if (resetSettings)
        Reset();
    checkAndMigrate();
    QSettings settings;
    
    setRestartRequired(false);
    
    
    if (!settings.contains("fHideTrayIcon"))
        settings.setValue("fHideTrayIcon", false);
    fHideTrayIcon = settings.value("fHideTrayIcon").toBool();
    Q_EMIT hideTrayIconChanged(fHideTrayIcon);
    
    if (!settings.contains("fMinimizeToTray"))
        settings.setValue("fMinimizeToTray", false);