Use a rocks glass when the drink is compact: an old fashioned, a small sour, whiskey over ice, or anything you want to smell as much as sip. Use a highball when the drink needs height, carbonation, or lots of ice. If cabinet space is tight, keep six of each before buying specialty shapes. That small set handles weeknight drinks, mocktails, spritzes, and water for guests.
A rocks glass is short because the drink is usually concentrated. It gives you room for one large cube, a twist, and a nose close enough to pick up spirit, citrus oil, or bitters. A highball is tall because the drink needs length. Ice, soda, tonic, ginger beer, and garnish all need vertical space so carbonation lasts and the drink stays cold without tasting cramped.
This is why the same whiskey can feel different in each glass. In a rocks glass, the pour reads as a slow drink. In a highball, the same spirit becomes lighter and more social once it is stretched with cold soda and citrus. Neither is better. They serve different pacing.
For buying, avoid novelty thickness and extreme shapes. A rocks glass should feel stable but not heavy enough to make a small drink look mean. A highball should be narrow enough to keep bubbles lively and tall enough for ice. If you are buying one set first, choose rocks glasses if you make old fashioneds and neat pours. Choose highballs first if your bar is mostly spritzes, mules, gin and tonics, or non-alcoholic long drinks.
Further reading: Bon Appetit on cocktail glassware, old fashioned glass background, and highball glass background.
