I have occasionally used the first phrase, but I have said it this way:
"I could not agree with you more"
This means "I agree with you". I don't often use the more wordy phrase above, but I will if I want to be very emphatic or dramatic.
The second phrase I have said this way:
"I beg to differ"
This is just an expression that means "I don't agree with you".
Hope this helps!