Is it better to buy an at-home smartphone based 6-lead ecg (like alivecor kardiamobile 6l) for someone with intermittent chest pain and arrhythmia, because by the time a 12-lead ecg can be done in a clinic, the episode may already be over?

Home EKG at the time of symptoms can be much better, depending on the quality of the home EKG, compared to the office EKG without symptoms. There are devices that record the EKG from days to months, that are attached to someone which is even better. In general, any testing, including EKG must be used in conjunction with seeing the entire clinical picture.

I would find it helpful if a patient was able to record even a 6 lead tracing during a bout of chest discomfort or palpitations. However it's important to call an ambulance immediately if you're experiencing what is perceived to be a serious set of symptoms that could represent a cardiovascular emergency and & not delay getting into the medical system as soon as possible.