Is prune juice typically a better choice for constipation than miralax (polyethylene glycol) is?

No. Especially at age 18 try not to be dependent on a laxative of any type except on rare occasions. Work on getting enough fiber (25grams/day) if you need help try a fiber supplement. Get pleanty of fluid (50oz of non alcoholic non caffeinated /day) and excersise especially anaerobic ....Hope this helps.