Session length and rhythm
Mario Kart: a race is about four minutes; a full cup is fifteen. It is easy to start, easy to stop, and easy for someone to hand over a controller between races. That makes it ideal for a party where people are also eating, talking, and arriving late.
Smash: a stock match is two or three minutes, which is even shorter, but the rhythm is different - it is a rotation game. With eight people you play, lose, hand over, and wait. Some groups love the tournament feel; others find the waiting deadening.
Controllers, and what each one needs
Mario Kart is unusually forgiving here: a single sideways Joy-Con is a completely viable controller because you only need accelerate, steer, and item. A Switch out of the box covers two players; two more Joy-Con cover four.
Smash is playable on a sideways Joy-Con too, but the button layout is genuinely cramped for a fighting game, and anyone taking it seriously will want a Pro Controller or a GameCube-style pad. Eight players locally also means eight controllers, which is a significant spend on top of the game - budget for it before promising an eight-player night.
Online
Both need Nintendo's subscription for online play, and both let you race or fight friends in other houses. Mario Kart's online is the smoother of the two and takes up to twelve players. Smash's online has always been the weaker part of the package - fine for casual matches with friends, frustrating for anyone who cares about precision.
If your group is spread out and online play matters more than local, Mario Kart is the better buy. If your friends are spread across different consoles entirely, neither works - read Switch + PS5 or Switch + PC instead.
Content and longevity
Smash has the larger package by a distance: an enormous roster, single-player modes, and a level of depth people have been exploring for years. If one person in the group wants something to master, Smash rewards that indefinitely.
Mario Kart's content is deliberately narrower and its replay value comes from the social loop rather than mastery. Both have been supported with substantial extra content - in Mario Kart's case a large paid course expansion, in Smash's case years of added fighters - so neither feels thin.
Which one for which room
Family with a wide age range. Mario Kart. It is the most inclusive multiplayer game Nintendo makes - see family games.
Six friends who all play games. Smash. Everybody is in, and the rotation is part of the fun.
A house where people drop in. Mario Kart. Nothing needs explaining.
A regular fight-night crew. Smash, easily.
Buying a gift for a Switch owner. Mario Kart, unless you know they already have it - see our gift guide.
If you can only buy one
Buy Mario Kart 8 Deluxe. It is the more universally useful of the two, it works with any group you put in front of it, and it is the one that still gets played five years later by people who do not consider themselves gamers. Smash is the better game for enthusiasts and the worse party purchase for a mixed room.
Live prices for both are in the table above. Full write-up of the winner is in our Mario Kart 8 Deluxe review, and the Switch pageranks both against the rest of the console's multiplayer library.