There are no real pros or cons here, at least none that should force a programmer comfortable in one language to have to use the other.
Performance shouldn't be an issue. It's unlikely that you'd write any heavy lifting with lots of messaging in the inner loops if you're a good Obj-C programmer, which means you'll really be writing those inner loops in C. If you're not a low level programmer, chances are the heavy lifting will be done by whatever libraries you chose to defer that too, and you'll be at the mercy of whatever they've chosen to do.
Portability is a real issue, if you care about it. If not, then meh. Being cross platform isn't the end all be all. Being successful on one first would be nice :) You can always port later. If you're not carefully planning to be cross platform from day 1 anyway, just being in the same language won't get you very far: there are plenty of further real differences between platforms that need addressing.
Finishing the project is more important that fretting about the tech, and if you're more productive in Obj-C then stay in Obj-C.
My preference? I'm a C++ guy. I like Obj-C just fine, but I'm happy in C++, and I do write cross platform code.