there's no benefit compared to the default image, except for the Python OpenCV bindings (which don't work yet on the default image and do work on eglibc one). So if you need OpenCV in Python - you need eglibc-based image and then you'd be better off using DevKit. If you don't need OpenCV in Python (or at all) - the default one (or the devtools one, based on the default) would do.
well, always depends on what you want to achieve. With eglibc being "embedded glibc" it is much closer to glibc than uclibc. If you want to bring a SW to Galileo - not only OpenCV - you might have to face quite some challenges adopting for uclibc whereas it probably would be a much smaller journey (or feasible at all) using eglibc.
Possibly you might also see differences in performance - haven't measured on G though.