'...I have read that the 6 lead one is hard sometimes to adhere to the knee or ankle. Quite a few complaints about that. Not sure whether to go for the card model or the basic one lead. Anyone have any thoughts?'

If it is a poor connection due to dermal contact, I would blame body hairs and/or skin oils. I am an apnea sufferer, and must wear a mask and use a PAP machine each night to stay healthy. In fact, the polysomnography was the very last test my cardiologist authorized, and it showed severe apnea, a complete surprise to me. The point is, I have to tape across my mouth or my slack jaw opens while I sleep and most of the flow from the machine escapes through my slack lips. This does no good for splinting my airway when I need to inhale or to exhale. In order to ensure good adhesion, even re-using the somewhat costly NexCare 'strong hold' tape that I use the next night or two, I wash my mouth with hot water and liquid hand soap before applying the tape. This ensures both good adhesion and reduces degradation of the adhesive so that I CAN use it at least one more night to keep costs down.

As windyshores relates, most of the time you can get away with the one lead. You'll still get a good determination of the nature of your heartbeat. If you ever wish to, or are asked to, provide a readout to an EP or cardiologist, or to take to the ER if you have a really bad bout of it, use the other leads, but take the time to prep the areas of application. Shouldn't take more than a full minute extra (which, I know only too well, can be an eternity when in arrhythmia...).