Personal thought: the rest of the Fez line will never disappear. There will always be a need for Arduino compatible pinout devices (sadly). Can’t see that need dissolving anytime soon, so can’t see GHI stopping something that is an enabler of people with ard-based shields migrating to a better platform.
Whether Spider or Cobra suits your needs better is a great question - what are your needs? (that’s a rhetorical question ) If you have a specific project in mind, and want to have that in it’s own little contained box, then the cobra is awesome for that. But if you are like me and you like connecting new things to your device and figuring them out, then perhaps Spider is more suitable. Really it will come down to space - Cobra contains the space a little more than Gadgeteer will as it uses bigger cables and connectors, where Cobra can be quite compact; and the second aspect is do you want plug and play (and then unplug and plug in new stuff) or do you want to assemble what you have more permanently.
Over time I expect that Gadgeteer based devices and sensors will become another “defacto” standard. As that happens, you’ll see more benefit in using a Gadgeteer based module. You won’t have the challenges in the hardware as much, and there’s software benefit too - the whole concept is to easily allow you to prototype a solution, hardware and software, much quicker than ever before.
E-blocks are really a simple connection method that has minimal wires. Gadgeteer extends that to be able to support many devices (because it has more signal wires) but the downside is that there’s more space consumed.
The other aspect is software. Because Gadgeteer devices “know” what type of device you can plug into any particular port, and you have an additional SDK, you are slightly further abstracted from knowing how to deal with some of the concepts that you deal with today. Take for example the camera capability shown off with the flipbook maker - the sample code Gus showed was a handful of lines only, trivial for even a poor coder like me to use. But if you want the full flexibility of writing your own code (with the downside of also having to deal with the hardware side more) then perhaps a Cobra would be better.
So in summary, “it depends” (a classic consulting phrase that I never get tired of repeating :))