Yes, there is a difference between an ECG and EKG here in the U.S. Ask any cardiologist. An ECG is the term used to describe monitoring of the heart, while an EKG is the term used to describe the diagnostic acquisition of the heart waveforms.
An ECG is usually a 3 or 5 lead waveform monitoring, while an EKG is usually a 10 or 12 lead system that captures enough information off the electrical signals of the heart to perform a more accurate diagnosis. It is a subtle difference but from a clinician's standpoint, do not ever use the incorrect term when talking to a cardiologist.