Yes I agree - Owen's poetry lacks the bitterness that is in Sassoon's poems.
Another thought: the final poem in Sassoon's collection (in my edition anyway) is "Everyone Sang" which is one of his really famous poems. It is very beautiful, but somehow it seems quite separate from all his other poems about the war - a sudden "happy ever after" sort of ending after all the grim realities of what they had all been through.
Perhaps he felt like that in April 1919, the first spring of the peace. Then as time went on, he became more cynical and bitter because of the aftermath of the war and the glorifying, as in the Menin Gate poem you quote above, fantasyfan.