Both of these allows reading the data without locking the target row. But Read uncommitted allows you to read changes which is not committed yet also known as Dirty read which is something to avoid in prod environments in most cases. Snapshot isolation makes the use of row versioning without waiting for the transactions that may modifying the rows but returning committed data. Preferably this will be your friend unless your app can have dirty reads in that case you can use Read uncommitted. You may read https://learn.microsoft.com/en-us/sql/t-sql/statements/set-transaction-isolation-level-transact-sql?view=sql-server-ver16