Is There a Difference? Which Should I Use?
Talk and Huddle are actually pretty similar to each other in the concept that they help you keep in contact with people like a chat program would. Both are capable of single or group chatting, allow you to speak to your contacts from G+ and your Google contacts, and of course, both are free for use.
There are some big differences though. For one, Huddle cannot be used on the desktop (at least not yet); it's a strictly mobile application which is restrictive. Talk can be used on both the desktop (in both web form and desktop app) as well as mobile.
Most likely, you'll have both apps on your Android phone, at least if you go ahead and download the G+ app from the Android Market. So what situations would you even use these?
Talk - Talk is best used as a chatting platform, for when you're taking a very long road trip (when you aren't the driver), or if you see a friend that happens to be on. I'm sure more people will use the computer version more than the mobile version, but one never knows.
Huddle - Huddle really works best when you're meeting a group of people and plans change. For instance, let's say that Marla is the first to reach your decided upon restaurant, but finds there's an hour wait to get in. Without huddle, she might just need to send out texts to every person, who will of course respond to her and not each other. This means she'll have to rely everyone's thoughts to everyone else.
With Huddle, she then quickly gets on to G+, goes to Huddle, (there's also a separate Huddle app that comes with the G+ app) and invites the people she is having dinner with and tells them about the situation. Instead of several text messages going out, they're all condensed in one space, like a group chat. Thus everyone knows there's a change of plans and everyone can decide where to go next.
Because G+ is still in beta, there's no telling what the final outcome could be. Personally having both Talk and Huddle seems redundant on a mobile phone, however there is talk that some Google Plus users would like to have a Hangout feature that would allow for video and voice chat. This is something that Talk is capable of; could Talk be merged in with Huddle?
There's no telling what the next few days and weeks will spell for both applications.