THE SHORT VERSION
If you have never solved either: start with Sudoku. Not because Killer is harder — it isn't always — but because Sudoku teaches you the spatial reasoning you'll need to use cage arithmetic effectively. Killer Sudoku without Sudoku fundamentals is just arithmetic in a void.
If you can already solve easy and medium Sudoku: try Killer next. You have exactly the technique you need. The cage sums will feel like a new vocabulary for the same language, not a different subject.
If you're an experienced Sudoku solver feeling bored: Killer Sudoku is where you go. It removes the starting digits entirely and replaces them with arithmetic constraints. Some expert Killer puzzles are harder than any Sudoku equivalent — but they get there through elegance, not frustration.
WHAT THEY ACTUALLY SHARE
A 9×9 grid. Every row, column, and 3×3 box must contain the digits 1–9 exactly once. Those three rules are identical in both puzzles.
Every Sudoku technique you learn — naked singles, hidden singles, pointing pairs, naked pairs — works exactly the same way in Killer Sudoku. The technique doesn't change. What changes is how you generate your starting candidates.
In Sudoku, starting candidates come from the given digits. In Killer, they come from cage arithmetic. Once you have a working candidate list, Killer Sudoku is Sudoku.
WHY KILLER ISN'T ALWAYS HARDER
This surprises people. An advanced Sudoku with only 17 given digits can be brutal — naked pairs and hidden triples and X-wings from the first cell. A well-constructed Killer at the same rating opens immediately from unique-sum cages.
A 2-cell cage summing to 3 can only be {1, 2}. A 2-cell cage summing to 17 can only be {8, 9}. These cages are arithmetic equivalents of given digits — sometimes more constraining than a scattered handful of pre-filled cells.
The 45-rule extends this further: any complete row, column, or 3×3 box sums to 45. Once you know that, whole regions of a hard Killer puzzle open up instantly. An equivalent Sudoku has no such shortcut.
THE CASE FOR STARTING WITH KILLER
There is one. Some solvers find Killer more approachable at the beginner level because the arithmetic constraints are explicit. You're not scanning an opaque grid for hidden singles — the cage tells you directly that cells A, B, C sum to 12. That's something concrete to work with.
If arithmetic feels natural to you and you find Sudoku scanning dry, start with Killer. The two puzzles will teach you different entry points to the same spatial reasoning, and some people click faster with arithmetic clues than with positional scanning.
The main risk is that Killer's endgames require Sudoku scanning. If you start Killer and find yourself stuck in the back half of every puzzle, the missing piece is almost certainly a Sudoku technique, not a Killer one.
THE HONEST ANSWER
Play both. They're the same game with a different front door. Sudoku builds the scanning muscle. Killer builds the arithmetic deduction muscle. Experienced players who alternate between them consistently solve both faster than players who specialise in one.
If you have to choose one to start today, make it Sudoku. You can reach Killer in a week if you're motivated. In GridJoy, both scale from Initiate to Zen Master — five difficulty tiers, procedurally generated so you're never solving the same puzzle twice. Start at Initiate in whichever you pick and work up. The technique will follow.