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.

return settings.value("fUseSeparateProxyTor", false);
        case ProxyIPTor:
            return GetProxySetting(settings, "addrSeparateProxyTor").ip;
        case ProxyPortTor:
            return GetProxySetting(settings, "addrSeparateProxyTor").port;
#ifdef ENABLE_WALLET
        case SpendZeroConfChange:
            return settings.value("bSpendZeroConfChange");
#endif
        case DisplayUnit:
            return nDisplayUnit;
        case ThirdPartyTxUrls:
            return strThirdPartyTxUrls;
        case Language:
            return settings.value("language");
        case CoinControlFeatures:
            return fCoinControlFeatures;
        case DatabaseCache:
            return settings.value("nDatabaseCache");
        case ThreadsScriptVerif:
            return settings.value("nThreadsScriptVerif");
        case Listen:
            return settings.value("fListen");
        default:
            return QVariant();
        }
    }
    return QVariant();
}
bool OptionsModel::setData(const QModelIndex & index, const QVariant & value, int role)