Although it's been answered, to expand on it a bit further and give a brief overview of both games, Super Mario Party and Mario Party Superstars are two different games and, while they share the same overall goal of "get the most stars/coins to win", are mechanically different. Super Mario Party came out first and only allows use of the Joy-Con for its gameplay + minigames, on top of the characters all having their own exclusive dice (these aren't fully balanced, some are better than others) and being able to get allies during the game. I know there's some game modes other than the basic Mario Party mode, but I'm unable to really comment on them. Admittedly, I haven't played it myself, so my knowledge of the game comes from the gameplay I've watched of it.
Mario Party Superstars came out 3 years later and is more closely aligned with the earlier entries in the series, so just the one dice (1-10 instead of 1-6), no allies, stars cost 20 coins instead of 10, and all controllers can be used instead of just the Joy-Con. Its boards/minigames come exclusively from older games, where it has some remade boards from 1-3, and some remade minigames from 1-10. You can save the games and resume them at a later time, and it is more customizable in some aspects (i.e. can turn on only certain minigames, can choose whether or not bonus stars come into play, can add turns during the game). Its online is also better than SMP since it does allow for disconnects/reconnects. That said, it does feature a smaller character roster (10 vs SMP's 20), and its game modes outside of the Mario Party mode are lacking in comparison to SMP.
Personally, I'd get Mario Party Superstars, since I feel like it's the better game of the two and better captures the essence of what Mario Party is, while still being something that newcomers to the series are able to pick up and play without too much trouble and end up enjoying IMO.
