Never really saw much G.I. Joe, we had another clone show on TV when I was a kid, it wasn't Action Man (that was much later, I think), but something with Force in the name, I think. I didn't catch much Transformers either, just a couple of episodes, and remember thinking those blue shining blocks where pretty cool, but the animation where they just disappeared when they stacked them on top of each other was dumb, and that's about it. For the most part I grew up without access to commercial television, as that stuff was relatively new at the time, and was too expensive for my parents, so I just had access to the Swedish public broadcast channels, so most of the stuff I saw was less action figure/toy oriented.
I do remember watching Dino Riders and Turtles though, but that was a bit later when a neighbor had subscribed to the commercial channels, and we got some bleed over to our TV jack, with really grainy reception of those channels.
Why not both?
Seriously though, GI JOE all day long. I love Transformers, always have. But my formative years just wouldn't have been the same without Snake Eyes. I remember getting the series Ten version and flipping my shit, he had RED FUCKING GOGGLES.