Just to add clarification in case others come here looking for information:
JTVCLIENT is the same as TWITCHCLIENT, but it’s best to use TWITCHCLIENT going forward.
TWITCHCLIENT by itself is the same as TWITCHCLIENT 1, and allows you to get userdata PMed to you by a user named “jtv.”
TWITCHCLIENT 2, however, is what the Twitch website uses nowadays. The big difference is that TWITCHCLIENT 2 prevents you from seeing channel joins and parts.
It is rumored there will eventually be a TWITCHCLIENT 3 that will change userdata to be PRIVMSG’d to the channel it originates, rather than by private message, providing channel context to the userdata. It is unknown if Twitch will ever implement any type of channel context to userdata, but it is heavily desired.