Umm, I believe they were both fission bombs. One was uranium, the other plutonium. Both those atoms are heavy and release energy when split. The lighter atoms, such as hydrogen release energy when they fuse. If you've ever seen a graph of
nuclear binding energy you'll see that iron is the most nuclear stable element, with atoms lighter than iron releasing energy during fusion (and hence taking energy to split), and atome heavier than iron releasing energy during fission (and hence taking energy to fuse).
At any rate, I believe this to be possibly irrelevant to the question of igniting Jupiter. Since Jupiter is mostly light elements, it would be powered by nuclear fusion. From what I understand it takes heat and pressure to ignite such a reaction, and the heat released by the reaction would probably be able to maintain it provided the pressure remains sufficient and the fuel source doesn't run out. As Jupiter has no shortage of fuel, and I believe the heat from either type of nuclear device could be sufficient, I suspect pressure would be the major factor. Jupiter certainly is big, with lots of gravity, and I assume lots of pressure, but I'm not sure it's enough. After all, it hasn't ignited already. Also, the pressure would be greatest at the core, and I expect any probes crashing into Jupiter would burn up long before reaching the core, somewhere in the upper atmosphere where pressure isn't as great.
Now, if a probe was crashing into a giant planet made of uranium, which for some reason was significantly pure, despite natural tendencies, I suppose it might matter if the probe died a firey death or a firey/neutron death. In which case the nature of the reaction might matter. Although, both fusion and fission reactions can release neutrons, from what I've read. But, this case of a planet of fissile uranium (or plutonium) would seem to be far less likely. Actually, the more I think about it, the more ludicrous it's starting to sound. Especially since the density of a planet wouldn't be uniform, and density matters here for whether the reaction is spontaneous or not. Again, if the core hasn't already ignited, then the surface probably isn't anywhere near as dense as needed to sustain a reaction. Further, the heavier elements would probably end up in the core, making uranium an unlikely crust. At best you'd probably be talking about a small asteriod chunk of uranium, which wouldn't have enough gravity to hold itself together after igniting.
In summary, from what little I know of nuclear chemsitry and the formation of stars, I somehow doubt we'd be igniting any gas giants by accident, and probably not even if we tried. Mind you, it would be cool to try it and see (in someone else's solar system).
