I used to wonder why we spelled “till” with two Ls when there’s only one L in “until.” Turns out “till” isn’t actually a quirky shortening of “until”; it beats “until” to the punch by about three centuries. Whoops.
Here’s an in-depth consideration from Merriam-Webster.
Basically, they mean the same thing, but “till” came first and is the probable source of “until.”
Either one gets the job done, but you came here for advice, so I’ll weigh in on usage.
I err on the side of aesthetics, i.e. you should choose the one that results in better alliteration, assonance, or Spoonerisms. To wit: Wait till Tim’s finished. “I’m not giving up until I find the right one,” said Tim. Looks like he’ll keep licking toads till he trips balls.
You could make an ecological argument that “till” uses less ink and takes up less space on the page than “until.” But if you find that kinda of argument compelling, you’re probably too busy saving the world from circumpolar culture to be bothered.
In terms of mechanical energy efficiency, “till” is the clear winner because it takes one less keystroke and the double letter requires one less “return to home row” trip. Speedrunners take note.
I’m neither qualified or informed enough to help you with medical considerations, but it seems like a split decision if you’re trying to manage Carpal Tunnel Syndrome. As discussed above, “till” requires one fewer keystroke than “until,” however “until” breaks up the flow between left and right hands more evenly and doesn’t require a dexterous double tap.
Oh, and if you want to shorten “until,” use “’til” — with a single apostrophe that’s basically impossible to see because of those double quotes. It’s not part of the preceding considerations because anyone who’s willing to employ apostrophes for dialectical speech has bigger fish to fry than humor, conservation, or medicine.
Speaking of populism, at least as far as the printed word goes, Google Ngram shows a decided preference for “until” beginning in the mid 19th century.
One last thing. Cambridge Dictionary parrots the erroneous assertion I began this diatribe with. They’re wrong — again, demonstrably wrong — but they’re the ones shilling and proctoring English exams.
Maybe just listen to them until the exam’s over.