TiVo Series3's require two cable cards, either "S" or "M". TiVo HD/HDXL's can use a single "M", multi-stream or two "S", single-stream cable cards. Initially VZ and other cableco's only offered "S" cable cards. Many posts are now indicating that VZ is starting to deliver "M" cable cards depending on the geographical area. "M" cards can decode up to six channels simultaneously. Of course TiVo is only capable of processing two. "S" cards only decode one video stream. The setup process takes a little longer with "S" cards because they both have to be activated/paired separately (slot 0 first, then slot 1). That said AFAIK there's no technical advantage to having an "M" card other than removing one failure point. (But if there is I'd also like to know what it might be). As you mentioned, the only advantage to using one "M" over two "S" cable cards that I'm aware of is the possible reduction in cost to the final user. Some companies charge per card. Sometimes the first card is free, then from as little as $1.50 to $7.00 for each additional card.