I like Quadraxas' code... unlike your second code with the two ifs, his pXent_addforcecentral is called a maximum of once even if both keys are pressed, whereas yours could be called twice if both keys were pressed.

I largely agree with ventilator though. When was the last time you experienced a slowdown because of too many ifs in a function? Or too many vec_sets? Worry about 65 entites who while(1) c_trace 5 times per frame.