Both poems do address death, but with very different approaches. The tone of “Wake Not for the World-Heard Thunder” is soothing, and death is a reward for hard work and courage. In 'Thou art indeed just, Lord, if I contend' on the other hand, the tone is angry and frustrated. Death is not a welcome reprieve, but a failure both of the speaker and God.
In “Wake Not for the World-Heard Thunder” the speaker seems to be comforting soldiers to resign to death. Consider:
Fear not you for flesh or soul;
Marching, fighting, victory past,
Stretch your limbs in peace at last. (lines 6-8)
Even more specifically, he tells them they did their best and have earned their death. Here's an example:
Times enough you bled your best;
Sleep on now, and take your rest. (lines 15-16)
The speaker is begging God, at the beginning of the poem. He seems to feel that he has lived his life well, and is only disappointed. He seems to feel defeated by God.
In 'Thou art indeed just, Lord, if I contend' the speaker is not satisfied. He believes that he has worked hard and God has not helped him.
Why do sinners’ ways prosper? and why must
Disappointment all I endeavour end? (lines 3-4)
The speaker has given up. Consider this line:
birds build – but not I build; no, but strain (line 12)
The speaker feels like he has no interest in rebuilding what he has lost.
So, as you can see, death is treated quite differently in the two poems even though they have the same theme of death. In one poem, death is a reward and there is no shame. In the other, death is defeat and cause for pain.