SQL Server Version Store Performance differences between RCS and ASI I understand that Read Committed Snapshot (RCS) operates at statement level, and Allow Snapshot Isolation (ASI) operates at transaction level, and both store row versions in tempdb. I also understand that ASI must be enabled in the DB, and in the settings of each connection.

My question is concerning the level of load that is placed on tempdb under RCS versus ASI. For example, for the same workload, would the load on tempdb be similar, or would the load on tempdb typically be higher (because it has to store more row versions)?
Just turning on the database setting will affect for how long the versioned rows need to stay in tempdb (since transactions that could be running in true snapshot would have repeatable read). I.e., enabling ASI will keep row versions for a longer time than enabling (only) RCSI.

As for performance for the applications we can't say. Only enabling ASI doesn't change client application behaviour unless they also explicitly ask to run with snapshot isolation. I.e., doesn't change default behaviour for apps. 

Assuming that all your apps consistently are modified to use the snapshot isolation (pretty unlikely) then the performance will be pretty similar between the two. Readers aren't blocked by writers. You can get into edge cases with update conflicts and how they are handled with error handling, re-try etc. But there are so many dependencies here, so that cannot be generalized - and those are edge-cases anyhow so it is highly unlikely that they will have a measurable impact.