A rocks glass and a highball glass are not the same object. One is short and wide. The other is tall and built for whisky, ice, and soda. If you want one first glass, match the form to the drink you pour most often.
EDO 1834 sells the short form as a whiskey glass and the tall form as a tumbler. The names in shops vary. The silhouettes do not.
This guide says when the rocks glass is the right first buy, when the tall glass is better, and how a small set can hold both.
Two shapes, two drinks
A rocks glass is a short vessel with a wide mouth. It is for whisky neat or on a large cube. The drink stays in a compact column. The hand holds a low weight. Aroma stays near the rim. Ice, if you use it, is usually one piece.
A highball glass is a tall tumbler. It is for whisky and very cold soda over stacked ice. The height gives the bubbles room and keeps the mix cold as you drink. The pour is longer. The glass spends more time in the hand.
Japanese highball service is a cold glass, dense ice, whisky, then soda, with a gentle stir. That drink does not belong in a short rocks glass if you can choose. The soda will crowd the rim or force a small, flat mix. The rocks glass can hold a short whisky and soda in a pinch. It will not give you the same drink.
When the rocks glass is the right first buy
Buy the rocks glass first if you drink whisky neat or on ice. That is the more common home serve. One mid-size rocks glass will cover a weeknight pour and a guest. You can add a tumbler later if the highball becomes a habit.
Choose interior width for the cube you use. Choose color from the room. Leave height to the tumbler. A tall rocks glass is not a highball. It is a large short glass, and it will still sit wrong for soda.
Read How to Choose a Japanese Whiskey Glass for form, size, color, and cut. Read What Size Whiskey Glass Do You Need if ice is the part that is stalling you. Shop whiskey glasses for the short form. The First Pour is the opening edit if you want a shorter list.
When a tall glass is better
Buy a tumbler first if the drink you want is a highball, water, or another long pour. Height is the point. Ice can stack. Soda can sit. Water at the desk looks correct in a tall glass and oversized in a rocks glass.
Chill the tumbler if you are making a highball. Use dense ice. Pour the whisky, then the soda. Stir once, gently, so the bubbles stay. A common home ratio is one part whisky to three or four parts soda. The glass should have room above the drink so you can carry it without the rim wetting your hand.
Clear and Mist Blue keep a highball looking cold in daylight. Smoke and Cobalt hold a night shelf. Night Water is the room if you want the tall, dark edit. Shop tumblers for the tall form.
Building a small set
If you can buy two glasses, buy one rocks glass and one tumbler. That pair covers neat, ice, water, and a highball. Match the color if you want a set. Mix clear with one dark color if you want the drink to show in one glass and the shelf to show in the other.
Do not buy a highball glass and expect it to replace a rocks glass for a neat pour. The whisky will sit at the bottom of a tall well. Do not buy a rocks glass and expect it to replace a highball glass for whisky and soda. The mix will be short.
A sake glass is a third form. It does not settle this choice. If the bottle is whisky, you are choosing between short and tall, not between whisky and sake. Read Rocks, Tumbler, or Sake Glass: Which Form First if you are still choosing among all three.
Start with the drink in the glass tonight. Then buy that form. Add the other when the habit changes.
Related reading: Old Fashioned vs Rocks Glass · What Size Whiskey Glass Do You Need · Whiskey Glasses · Tumblers · The First Pour