Should I get a backpack cooler or a shoulder-strap cooler for real hiking?

Get the backpack. For actual hikes over a mile or with more than a few cans, a backpack cooler keeps your hands free and distributes weight on your hips, not your shoulders. A shoulder-strap cooler is fine for short walks or beach days, but on a trail it digs in and swings around. Let your shoulders tell you which one to grab.