They are neither plants nor animals.
They differ from other forms of life such as the micro-organisms and the fungi. Instead they resemble the primeval body of life and are generally known as Mushi. Their existence and appearance are unknown to many and only a limited number of humans are aware of them.
I really don't know the power levels on both of them, but I would guess their both pretty much equal. After all, both warriors are Goku and Vegeta fused. As stated before, Goku is dominant in Gogeta while Vegeta is dominant in Vegetto.
Edit: I may have to do some more research into this. But Super Saiyan 3 Gotenks is stronger then Super Saiyan 3 Goku if you compare powers. It may just be that the multiplier was doubled (the ss3 multiplier of power) but I think that a fusion adds power to the warrior. Some fusions may give more power then others, so Gogeta may be more powerful then Vegetto or vice verca depending on which form provides more power.