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.

{
    if(role == Qt::EditRole)
    {
        QSettings settings;
        switch(index.row())
        {
        case StartAtStartup:
            return GUIUtil::GetStartOnSystemStartup();
        case HideTrayIcon:
            return fHideTrayIcon;
        case MinimizeToTray:
            return fMinimizeToTray;
        case MapPortUPnP:
#ifdef USE_UPNP
            return settings.value("fUseUPnP");
#else
            return false;
#endif
        case MinimizeOnClose:
            return fMinimizeOnClose;
        
        case ProxyUse:
            return settings.value("fUseProxy", false);
        case ProxyIP:
            return GetProxySetting(settings, "addrProxy").ip;
        case ProxyPort:
            return GetProxySetting(settings, "addrProxy").port;
        
        case ProxyUseTor: