thanks @BertL for your detailed and thoughtful reply.
I purchase and typically keep my cars a long time (my last 2 were each ~15yrs), and I also follow recommended maintenance required by the warranty. My wife will still have her ICE vehicle but I imagine we'll use that less and less once I get my MS. But still overall I expect to be pretty low annual mileage for the two cars, maybe combined total will still be a little under 20,000km(12,500mi), unless we start doing more long roadtrips with the MS. Too early to say how long I will keep the MS, but at least 4yrs, possibly 8yrs, so I need to consider the available service plans and also possibility to extend the service agreement in addition to the service plans.
I'm guessing there may be some differences between the service plan T&Cs between here in Canada and the U.S. - I'm reading the Canadian agreement and it seems to say I can buy the +4 extension to the pre-paid service any time no later than 30days after the expiration of the original 4 yr prepaid service plan. However in the service section of MyTesla page, it says the 4yr extension plans are "only available in combination with an initial four year service plan at this time. Extension plans may be available for individual purchase in the future". Seems to be contrary to what's written in the agreement itself that can be downloaded from the same page, although the PDF seems to have a "032013" date in the footer. I also notice that the service plan is transferrable to a new private owner, but is subject to a $100 transfer fee. But good to know it can be transferred, I wasn't aware of that before you mentioned it. The PDF also clearly says that wheel alignments ARE included in the service, but again maybe the PDF is out of date if the discussions in TMC forums on that particular topic are any indication.
anyhow thanks again for your comments, some good things to consider.