Energy from radioactive decay is very low (it's derived from the weak nuclear force). It's generally been used to power things like spacecraft that need a tiny source of energy for a long time.
Fission on the other hand produces a large amount of energy (derived from the strong nuclear force). For fission you need to use fissile or fertile elements like uranium, thorium, and plutonium.
The difference in energy produced by these two means is tremendous. In fact, even newer spacecraft will be using fission.
Actually, fission energy comes from proton to proton repulsion, thus it is example of the awesome power of the electromagnetic force.