im thinkimg about purchasing either a KK bogi or a glider for my 4 yr old, to help him transition out of a snaffle because he is getting pushy in that bridle (its a PC loose ring with a smooth two piece)... 

out of those options which one would you choose, and why? or alternatives? TIA ????

Two totally different bits. The Bogi will be a much more of a direct bit because it lacks a lot of gag, while the Glider is kinda of like the name suggests and it has quite a bit of gag. If you're looking for the closest transition from an O-Ring, I would go with the Glider.