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 getMinimizeToTray() const { return fMinimizeToTray; }
    bool getMinimizeOnClose() const { return fMinimizeOnClose; }
    int getDisplayUnit() const { return nDisplayUnit; }
    QString getThirdPartyTxUrls() const { return strThirdPartyTxUrls; }