The first thing to note here is that you left out all the important information. The size of the machine, the size of the steppers, their voltage rating and inductance values are all important in selecting proper electronics. Right now we know nothing about the steppers themselves so no learned opinion on the suitability of these boards can be made.
Like wise a quick trip to one of the sites selling CNC Shield indicates a similar lack of info. Specifically what is the voltage rating of the drivers and current capacity. Frankly such info should be front and center. Beyond that there can be advantages to using discreet drivers.
Most likely this would be on more limited machinery which brings up the question of what usage are you targeting? For example a router for light duty use, say PCB routing might be well served by CNC Shield. A larger router targeting general wood working might be better off with a different solution.
This is the least of your worries when it comes to getting decent performance out of your implementation.The main thing I see is the "CNC shield" has replaceable Stepper Motor Drivers and the Gshield has the Stepper Motor Driver soldered to the board.
I like the idea that if I screw up and fry a Stepper Motor Driver that it can just be pulled out , replaced and back to cutting ( yes figure out why it fried first).
Well more information would be nice!!!! In my opinion such boards are only really suitable for very light duty machine implementations. We are likely talking size 17 steppers at a max. In any event not knowing more about your router concept it is hard to provide a decent answer as to such boards working at all.
Is there anything else I should think about ?
PS....it seems the Shapeoko community uses a Gshield , and was supplied with the Shapeoko kit
Thanks for your ideas
Shapeoko is a bit light as far as machine tools go. I would most certainly look for alternative router designs especially if you are going to build one your self. Going the DIY route will enable you to build a significantly better machine without totally blowing out the budget. By the way I'm not saying the Shapeoko can't do the job of a router, rather is is a light weight solution that is rather expensive compared to what can be accomplished DIY. If you are talking about the same model I am, the $1500 price tag is a bit stiff for a machine that isn't as stiff as could be had via a DIY project.
So my suggestion would be to look at some of the various build threads in this forum and more importantly read the sticky threads. This should give you an idea of what various people have accomplished with a wide range of experience and tooling access. A Shapeoko class machine could also be built cheaper going the DIY route. In any case I would urge you to consider what you need or want from the machine. Expectations are everything. I just don't want you buying electronic hardware until we know the hardware will fit the application.