i am using premium themes there have all seo function "title,description,and keywords" so if i use only my themes option. please tell me which will be better for my blog yoast or themes seo option?
keywords means nothing for google, you can use yoast or all in one seo. if you are a professional, you can set your meta tags by theme codes yourself.
You should go with Yoast over your Themes own SEO features in my opinion unless it you're using Thesis.
You should use plugin instead of using theme's seo option. Because if you change your theme in future you will lose all Custom Meta Title, Description you wrote. But if you use WordPress SEO by Yoast or any other plugin to do the work, you won't face any problem even if you change your theme. Hope this helps.
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