The first method is of course faster in terms of CPU cycles because of two reasons:
1) The CPU doesn't have to check two times if a variable evaluates to true
2) The CPU only needs to call the function pXent_addforcecentral once, instead of two times. Calling functions isn't completely inexpensive as you first need to push the variables on the stack and then jump to a completely different memory region to continue the execution. Jumping back to the caller needs to pop a few variables from the stack and copy the result into eax, also not completely inexpensive

That said, it doesn't matter which way you choose as the time isn't measurable for a human (for really, really, high precision timers its measurable, but your brain will fail this challenge).