I would start with READ COMMITTED. It's the default isolation level and most of the time is sufficient. Do some load testing, if you do have lots of blocking (and maybe deadlocks) and you can't resolve them by tuning indexes or optimising queries, then consider READ COMMITTED SNAPSHOT.
The downside of READ COMMITTED SNAPSHOT is that it has an overhead and puts a lot of load on TempDB
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: Discussions on DB performance with occasional diversions into recoverability
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