If these are the only two choices you have, then the first one is clearly the external conflict. When the Trojans fight the Greeks, it is an external conflict. When the Greeks are hiding in the horse, it is not.
An external conflict has one character or group in conflict with another. This is clearly happening when there is a batttle going on.
When the Greeks are in the horse, there is no conflict. Well... Cassandra is inside Troy saying that bad things will happen and no one will listen and that's a conflict. But just in terms of the Greeks sitting there hiding in the horse, it's not a conflict.