They both mean the same - and are used in the same way - as 'yo también' in Spanish.

Whilst 'Me too' can be used as a response to any comment, 'So .... I' has to be conjugated according to the grammar of the previous sentence. Here are some examples:

I like pizza.          So do I.

I'm tired.              So am I.

I can hear music.  So can I.

I've finished.         So have I.

I saw that film.     So did I.

As you can see, the 'So...I' form is much harder to use, but it's great practice for your use of auxiliary verbs!

The negative forms are 'Me neither' and 'Nor ....I' or 'Neither ....I'.

I hope that helps.

  I think the phrase;  "I agree." 

You're welcome. By the way, if you have language questions like this, it's better to post them in the 'Answers' section rather than here on the Discussions pages.

Indeed it helps.

Thanks!