Both tests are FDA approved and available. You should discuss the pros and cons of each test with your oncologist.
The serum test seems more approproiate for HER2+++ survivors as it measures the HER2 cells in the blood.
HER2 Serum Test
I received some literature descriping the serum test yesterday, If you email me your address, I'll send you a copy to pass on to your onc.
Regards
Joe