Although I have not had any experience with either from what I heard discoid might be a better choice. They are smaller, which would mean you could feed them to a wider range of gecko sizes. Also, they lay eggs while the dubias give "live" birth. With the discoid eggs, I have heard people actually remove them and hatch them in a separate container. This way you have very small roach nimphs to feed to baby geckos. Discoid also breed faster. I have leopards and bearded dragons, and intend to breed both. Overall, roaches of any kind would be a better feeder because of the higher meat to shell ratio. Keep crickets in their diet plan to give some variety though.
Discoids are also live bearers and are very similar to Dubia roaches. I have kept both with great success but find that dubia roaches "seem" to reproduce better comparatively when both species are kept under ideal conditions. In terms of nutrition, it is the quality of your gutload, (or the quality of the food that you feed the roaches) which determines their over all nutrient content. Most of my geckos seem to prefer dubia roaches over discoids.
check and make sure if your state has any illegal species of roaches that its illegal to ship whatever illegal species there is. i know florida has most but 2 roach species that are not illegal which discoid is one of them. i dont know if florida is the only one with these regulations but its good to check and make sure just in case.
imo both roaches are popular choices for feeders. if you can order both then order a small amount of both and see which you like better and which your geckos like better and then continue to breed those. good luck