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.

case ThreadsScriptVerif:
            if (settings.value("nThreadsScriptVerif") != value) {
                settings.setValue("nThreadsScriptVerif", value);
                setRestartRequired(true);
            }
            break;
        case Listen:
            if (settings.value("fListen") != value) {
                settings.setValue("fListen", value);
                setRestartRequired(true);
            }
            break;
        default:
            break;
        }
    }
    Q_EMIT dataChanged(index, index);
    return successful;
}
void OptionsModel::setDisplayUnit(const QVariant &value)
{
    if (!value.isNull())
    {
        QSettings settings;
        nDisplayUnit = value.toInt();
        settings.setValue("nDisplayUnit", nDisplayUnit);
        Q_EMIT displayUnitChanged(nDisplayUnit);
    }
}
bool OptionsModel::getProxySettings(QNetworkProxy& proxy) const