Generally and in theory it's better to use just the outdoor antenna, as it's likely to get the best signal and less "home noise". Keeping your indoor antenna connected adds some noise and leaks some of the signal.
That said, when you are at a spot with good signal, a random piece of wire will work nicely as an antenna, things become more critical if you want to receive far away or weak stations. I'ld go for the "recommended configuration" with only the external antenna and try to find the limitations of the radio. Then you can try different antenna combinations; some people even have a hobby in building antennas.
