By Aaron Maasho

TOULOUSE, France (Reuters) - Ethiopian Airlines is weighing whether to expand its fleet with the 366-seat Airbus A350-1000 or the latest version of the 777 long-haul jet built by its traditional supplier Boeing, widely known as the 777X, its chief executive said.

Tewolde GebreMariam was speaking to Reuters after taking delivery of the first of 14 A350-900 jets to be placed in the airline's fleet, leased from Dutch-based AerCap. The aircraft delivered to Ethiopian has 343 seats.

"We have 14 of these A350 and we have more (Boeing) 787s. We are comparing the A350-1000 and also the 777X. Depending on which performs well out of Addis Ababa at altitude and high temperature, we are going to make that decision," GebreMariam said.

Asked how many extra aircraft the airline could order, he said, "between 15 and 20".

He did not say when the airline planned to make a decision.

"We want to know more about the A350-1000," he said.

Earlier this month, GebreMariam told Reuters the airline was considering whether to order 10 to 15 Boeing 777-8s, one of two planned models still widely known as 777X, the label used before they were launched on the market.

The 777-8 will carry about 350 people.

Until now, Ethiopian Airlines has mainly stuck to aircraft from Airbus's U.S. rival.

"The concern has always been not to diversify too much because different types add complexity to the operation and make the cost of operation very high, so it is not recommended. But we are growing," GebreMariam said.

"The fleet is around 80 now. So any fleet type on its own will be around 20: the minimum size to justify the training of new pilots and technicians. This is the right time. At the same time, for competition reasons, it is required."

Ethiopian Airlines plans to double its fleet by 2025 under a plan called Vision 2025, but Boeing will continue to dominate the airline's fleet as Ethiopian flies the short-haul 737 as well as the long-haul 787 and 777 models.

"They (Boeing) will continue to take the larger share but Airbus will continue to grow," GebreMariam said.

(Additional reporting by Tim Hepher; Editing by Elaine Hardcastle)