Having previously owned an iphone I am reluctant to consider buying another Apple product due to the highly restrictive nature of their hardware and software. I've nothing against Apple per se, just the inflexibility of their products.
To that end I don't think there is too much to gain from Windows RT as it's a similarly closed system which is only able to run apps downloaded from their app store (or whatever they are going to call it). True, some of the RT tablets may be more flexible than ipad in some ways, e.g. connectivity.
I'm very happy with Android as a phone os but not totally convinced that I want my main tablet (which I'm currently in the market for) to run it - although it
would provide an excellent selection of low cost devices for SD if an Android version of SD was available. Windows sounds like it might be the right tablet os, but it has to be full Windows 8 for SD. I expect there should be no issue running SD (planning AND navigation) on any of the upcoming full Windows 8 tablets like the Lenovo Thinkpad Tablet 2, Asus Vivotab (not vivotab RT), Dell Latitude 10, etc, as well as Surface Pro. Some of these should be less expensive than Surface Pro. Can anyone confirm that Surface Pro will have GPS by the way?
This should hopefully provide a selection of SD-friendly tablets for non Apple fans
Still not cheap, but remember if you want an ipad with onboard GPS you need to buy the 3G versions, meaning even an ipad mini will cost around £449 for the 32GB version!!
Think I'll wait until the new year to properly evaluate the Win8 tablets. In the mean time I may get something like a Galaxy Tab 2 10.1 for general purpose use (including wife & kids) as it can be had for as little as £180 with the current Samsung cashback 