The website says to drop in the office if you're interested in the project. That's exactly what I did. I saw the bug live and met some of the team. This device is impressive and there is a lot more just around the corner from this company.
Just from a quick glance the BUG seems to be faster over all and includes polished packaging. However, I don't like how the BUG dev kit looks to be very Java Eclipse centric. The Gumstix provides a complete buildroot build environment which is probably more conducive to serious low level hacking, if the bug provides this they don't make it obvious on their website.
The BUG is also twice as expensive at the Gumstix though it does have better specs.