I ended up writing in mode VI this week. Since there is no musical precedent for an English Responorial Psalm in the Mass, I don't feel obliged to stick with a mode based on tradition from the Gradual. That's a nice freedom to have.
You can see my work here, at Chabanel Psalms.
That said, it was a challenge to find something singable that would capture the rhythm of speech and a hopeful countenance, which I think this text requires. The way the antiphon reads is a bit clumsy, a bit stilted. Had I been asked, I would have suggested turning it around a bit and starting with "to the upright," and completing the sentence with "the just man is a light in darkness."
Now mode VI was tricky for the verses. Some Office tones, like I mentioned last week, lend themselves more easily to English. The good thing about English is that you can argue where the stresses are in a sentence or phrase. If you change the stress, you change the emphasis or meaning ever so slightly. I don't think this can be overcome, and I choose to look at it as a more positive aspect of English.
If I have done harm to the text and made it read (sing) like one of those lectors....who pauses....after.....EVERY.....word, do let me know. My goal is to keep the English flowing, with nicely positioned vowels and clean consonants. Give it a try.