I highly recommend MSTS (Mark Smith) Training out of OK.  His classes fill up fast but you can get on the wait list and a lot of guys do drop at the last minute.  In my humble opinion he is better and more affordable than some of the others out there.  All the classes helped me.  510/570 are more straight forward than the 653 for sure but all are difficult tests.  Much better to take a class and study for 4 or so weeks than to beat your brains out and just study for 2-3 months.  You'll make back the cost of your class/test first time out.  API certs are way more in demand than the CWI in my opinion and I do have both.  However, if you like new construction a CWI is a better bet.  I love new construction but doggone it I love the money in the oil patch a lot more!!  Good luck on the test man.  If you put in the time to study it ain't bad.  But you gotta log a minimum of 150 hrs of book time to be comfortable with any of the API exams as a rule of thumb.  Yes I said minimum. . . .