The airline industry is increasingly debating the future of a potentially promising niche between popular medium-haul aircraft, like Boeing's 737 or the Airbus A320 family, and the smallest wide-body or twin-aisle jets.

The gap includes future replacements for the Boeing 757, which halted production a decade ago.

United Airlines, a unit of United Continental Holdings (>> United Continental Holdings Inc), is the second-largest operator of the 180- to 240-seat 757 after Delta Air Lines (>> Delta Air Lines, Inc.).

Airbus Group (>> AIRBUS GROUP) has made inroads into an area traditionally dominated by Boeing Co (>> Boeing Co), with plans to increase the range of its largest medium-haul model, calling it the A321LR.

Boeing said on Monday it was discussing studies for a plane slightly larger than a 757 with more range.

"We are looking at both," Ron Baur, Vice President of Fleet at United Airlines, told a conference hosted by the International Society of Transport Aircraft Trading (ISTAT) "The A321LR looks pretty decent, but we will see what Boeing is going to do in the mid-market."

"It comes down to what Boeing is going to do," he said, adding the age of United's 757 fleet meant it could evaluate all options and was under no pressure to make a rapid decision.

United Continental's finance director told Reuters last week he would like to see a replacement for the discontinued 757, which "has a particular sweet spot in our network."

Colombia's Avianca (>> Avianca Holdings SA), meanwhile, raised questions over the performance of Boeing's latest wide-body jet, the 787-9 Dreamliner.

"Boeing is trying to sell us the 787-9. We are analyzing that option right now, but the performance is volatile," said Jose Yunda, Avianca's director of fleet management.

"If performance improves a little, we would like to take another look," he told the Istat conference.

Boeing had no immediate comment.

(Reporting by Tim Hepher; editing by Chris Reese, G Crosse)