Kendra Little has a cheat sheet for comparing the two optimistic concurrency levels:
SQL Server offers two flavors of optimistic locking for traditional disk-based tables: Read Committed Snapshot Isolation (RCSI), and Snapshot Isolation. They are each great tools to reduce blocking and make applications faster, particularly in databases that have lots of tiny reads and writes that need to be quick, or mixed-use patterns (lots of little reads and writes + larger reporting queries).
Both isolation levels are available in SQL Server Standard Edition and Web Edition as well as Enterprise. That’s right, they’re cheap and easy. Each of them are controlled by database level settings, and default to disabled when you install SQL Server.
The moral of the story: both of these are awesome, both have potential drawbacks, and both need testing. I’ve had good experiences with RCSI, but even then, maybe about 1% of procedures need specific locking hints (either NOLOCK or an explicit lock) to maintain previous application behavior and to deal with the problem Kendra brought up. Moral of the story: test, test, test.