One of the beautiful aspects of the SI is that there is so little to fail. The #3 (analog) is going to be your most fail proof because it's as straight forward as you get. You may well have an element go out in 6 years (easily replaced), but there is no reason to expect that it would. Elements can last decades sometimes. There are no seals to go bad. There's no glass to break. Properly cared for the SI should last a lifetime.
The 3D (digital) could very well have more failures as the PID unit is an electronic part on an appliance that goes outside and generates lots of heat and smoke. That said, properly cared for that should also last. Don't forget that the SIs come with a 3 year warranty, pretty much have replacement parts available for everything that can fail, and a company/owner that really stands behind his products.