There are many differences. The full client lets you take advantage of many Notes functions such as doc links, replication, subscriptions, etc. iNotes gives you some replication features but not out of the box. The browser is a familiar interface for most people, but it lacks some ease of use functionality that the full client has. Also, browser sessions require at least 2x the RAM on the server as a client session. Encryption will not work with web clients. Finally, you cannot use standard Domino clustering for HTTP traffic. You need the Internet Cluster Manager (ICM). Your choice depends on what you need and what you have today.
This was first published in July 2001