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 (!settings.contains("strDataDir"))
        settings.setValue("strDataDir", Intro::getDefaultDataDirectory());
    
#ifdef ENABLE_WALLET
    if (!settings.contains("bSpendZeroConfChange"))
        settings.setValue("bSpendZeroConfChange", true);
    if (!m_node.softSetBoolArg("-spendzeroconfchange", settings.value("bSpendZeroConfChange").toBool()))
        addOverriddenOption("-spendzeroconfchange");
#endif
    
    if (!settings.contains("fUseUPnP"))
        settings.setValue("fUseUPnP", DEFAULT_UPNP);
    if (!m_node.softSetBoolArg("-upnp", settings.value("fUseUPnP").toBool()))
        addOverriddenOption("-upnp");
    if (!settings.contains("fListen"))
        settings.setValue("fListen", DEFAULT_LISTEN);
    if (!m_node.softSetBoolArg("-listen", settings.value("fListen").toBool()))
        addOverriddenOption("-listen");
    if (!settings.contains("fUseProxy"))
        settings.setValue("fUseProxy", false);
    if (!settings.contains("addrProxy"))
        settings.setValue("addrProxy", GetDefaultProxyAddress());
    
    if (settings.value("fUseProxy").toBool() && !m_node.softSetArg("-proxy", settings.value("addrProxy").toString().toStdString()))
        addOverriddenOption("-proxy");
    else if(!settings.value("fUseProxy").toBool() && !gArgs.GetArg("-proxy", "").empty())
        addOverriddenOption("-proxy");
    if (!settings.contains("fUseSeparateProxyTor"))
        settings.setValue("fUseSeparateProxyTor", false);
    if (!settings.contains("addrSeparateProxyTor"))