Should I use one of them or both?

Well this is a double edged sword. While they will prevent spam bots and fake emails from registering it will also prevent people from registering and then maybe even posting if they don't verify.

So I tend to use this rule. If your forum is smaller - medium sized then forget about both them. When you get larger you can add the steps as you chooose since they will probably be really wanting into your site. And if you can deal with a bit of spam then your ok.

Remember the easier it is for guest to register the better in most cases!

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I see no reason to turn on image verification unless your site is being nuked with register bots.