G5A is portable will do dreamcast emulation well. ..poor battery life (maybe battery can be modded)
G7 is not portable in the same way, its light and broader with slight bump in resolution good pc gamestreaming... has a large battery capacity
Much i5s, well its an expensive and powerfull device, but I would not recommend it unless you want to play hardcore android games and it lacks buttons R2/L2..which is bad when it comes to emulation and some games.
shield is not portable or usable for long periods of time, its a nightmare holding that thing to play games for more than 30 mins for me. Its comfy on a bed, still its just not ergonomically friendly to use for emulators, the thick bezels make the screen seem tiny.