this letter won't show up in my CV, as it is unreviewed, and barely a paragraph. It's a new record for me, though, first rejected (someone misread it) then accepted all within 2 days.

And as basically a complaint about previous work, it probably hurts more than helps, as it generally pays to be positive, but the radon/heat anomaly claims were too outrageous to go unchallenged.

I'm not sure the venue matters so much as do the citations. If people refer to your paper on paleoclimate, it's a plus for you no matter where it is published.

I don't understand the student thesis comment. Just write up and submit the most incisive and useful work first, why wait for someone who doesn't care and for a less interesting paper. If he contributed intellectually, acknowledge him or make him a co-author on the first paper.