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.

StartAtStartup,         
        HideTrayIcon,           
        MinimizeToTray,         
        MapPortUPnP,            
        MinimizeOnClose,        
        ProxyUse,               
        ProxyIP,                
        ProxyPort,              
        ProxyUseTor,            
        ProxyIPTor,             
        ProxyPortTor,           
        DisplayUnit,            
        ThirdPartyTxUrls,       
        Language,               
        CoinControlFeatures,    
        ThreadsScriptVerif,     
        DatabaseCache,          
        SpendZeroConfChange,    
        Listen,                 
        OptionIDRowCount,
    };
    void Init(bool resetSettings = false);
    void Reset();
    int rowCount(const QModelIndex & parent = QModelIndex()) const;
    QVariant data(const QModelIndex & index, int role = Qt::DisplayRole) const;
    bool setData(const QModelIndex & index, const QVariant & value, int role = Qt::EditRole);
    
    void setDisplayUnit(const QVariant &value);
    
    bool getHideTrayIcon() const { return fHideTrayIcon; }