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.

{
    
    
    QSettings settings;
    static const char strSettingsVersionKey[] = "nSettingsVersion";
    int settingsVersion = settings.contains(strSettingsVersionKey) ? settings.value(strSettingsVersionKey).toInt() : 0;
    if (settingsVersion < CLIENT_VERSION)
    {
        
        
        
        if (settingsVersion < 130000 && settings.contains("nDatabaseCache") && settings.value("nDatabaseCache").toLongLong() == 100)
            settings.setValue("nDatabaseCache", (qint64)nDefaultDbCache);
        settings.setValue(strSettingsVersionKey, CLIENT_VERSION);
    }
    
    
    if (settings.contains("addrProxy") && settings.value("addrProxy").toString().endsWith("%2")) {
        settings.setValue("addrProxy", GetDefaultProxyAddress());
    }
    
    
    if (settings.contains("addrSeparateProxyTor") && settings.value("addrSeparateProxyTor").toString().endsWith("%2")) {
        settings.setValue("addrSeparateProxyTor", GetDefaultProxyAddress());
    }
}