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 successful = true; 
    if(role == Qt::EditRole)
    {
        QSettings settings;
        switch(index.row())
        {
        case StartAtStartup:
            successful = GUIUtil::SetStartOnSystemStartup(value.toBool());
            break;
        case HideTrayIcon:
            fHideTrayIcon = value.toBool();
            settings.setValue("fHideTrayIcon", fHideTrayIcon);
    		Q_EMIT hideTrayIconChanged(fHideTrayIcon);
            break;
        case MinimizeToTray:
            fMinimizeToTray = value.toBool();
            settings.setValue("fMinimizeToTray", fMinimizeToTray);
            break;
        case MapPortUPnP: 
            settings.setValue("fUseUPnP", value.toBool());
            m_node.mapPort(value.toBool());