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.

fMinimizeToTray = settings.value("fMinimizeToTray").toBool() && !fHideTrayIcon;
    if (!settings.contains("fMinimizeOnClose"))
        settings.setValue("fMinimizeOnClose", false);
    fMinimizeOnClose = settings.value("fMinimizeOnClose").toBool();
    
    if (!settings.contains("nDisplayUnit"))
        settings.setValue("nDisplayUnit", BitcoinUnits::BTC);
    nDisplayUnit = settings.value("nDisplayUnit").toInt();
    if (!settings.contains("strThirdPartyTxUrls"))
        settings.setValue("strThirdPartyTxUrls", "");
    strThirdPartyTxUrls = settings.value("strThirdPartyTxUrls", "").toString();
    if (!settings.contains("fCoinControlFeatures"))
        settings.setValue("fCoinControlFeatures", false);
    fCoinControlFeatures = settings.value("fCoinControlFeatures", false).toBool();
    
    
    
    
    
    
    
    
    if (!settings.contains("nDatabaseCache"))
        settings.setValue("nDatabaseCache", (qint64)nDefaultDbCache);
    if (!m_node.softSetArg("-dbcache", settings.value("nDatabaseCache").toString().toStdString()))
        addOverriddenOption("-dbcache");
    if (!settings.contains("nThreadsScriptVerif"))
        settings.setValue("nThreadsScriptVerif", DEFAULT_SCRIPTCHECK_THREADS);
    if (!m_node.softSetArg("-par", settings.value("nThreadsScriptVerif").toString().toStdString()))
        addOverriddenOption("-par");