One way in which Owen's poetry might have been different had he not been in military battle would lie in its depiction of war itself.
Wilfred Owen's participation in the war made him aware of its terrors. Experiencing these in a direct manner had a tremendous impact on his writing. Had Owen not been in the service, he might have depicted war in a "surface" manner. It might have been a simplistic rendering of war. Owen understood that millions of young men eagerly enlisted to fight because they were told of its glory. "Doomed youth" were told of how quick it would be and how their triumph was almost guaranteed. These boys were denied a vision of its true terror. Owen realized that the claims of nationalism, patriotism, and honor rang hollow when said by individuals who had not experienced "the pity of war."
Owen was able to hone his poetic voice because of his war experiences. His stay at Craiglockhart military hospital enabled him to use his writing as a therapeutic release. It also allowed him to befriend Siegfried Sassoon. This enabled Owen to further develop his writing about war from a uniquely personal and thematic frame of reference. This artistic growth does not happen had Owen not been to war. His thought and writing are reflective of experience.
Who is Siegfried Sassoon?
He belongs to stream of poets known as War Poets so of course his poems would be different. Every poet writes from his experiences and his own thoughts regarding those experiences, if he hadn't been to war maybe he would write love poems or what else.