I use both on my droid team. 86 hits harder and crits more often, Geo is faster, has more hp, but doesn't hit quite as hard. Both have specials that call assist and 86 gets a damage bonus if he calls another droid, while Geo gets a damage bonus if he calls Poggle.
Geo also gets extra crit chance and turn meter gain on a crit from his unique. 86 gets more critical chance for each droid ally, and more damage from his unique.
86 is a better choice for a droid squad although Geo is not a bad choice either. Geo fits good into any team where 86 really needs other droids to shine.