My experience is that road shock is a bigger enemy to your computer than an imagined crash. I go with a backpack. On bad potholes you can get off the saddle and cushion the blow with your legs. In general, anything trying to shake your computer has to shake you as well, so your mass acts as a shock absorber.

This is not just anecdotal. I used to use my bike to carry military spec electronics as part of my job. The failure rate of chips carried on my bike was much higher. Chips carried in my backpack never failed.