Depends. if you care about mileage and your truck is otherwise relatively stock, I'd go with the 32/36.
i run one on both my 84 and 82 and didn't notice a big reduction in mpg on either but definitely gained a good amount of power and torque also. the 32-36 are primary-secondary in nature meaning you will do about 80% of your driving on the 32mm throttle, once you floor it, the 36 plate will open and give a noticable "kick" to the engine
38/38; used to be called "outlaw" kits; both 38mm plates open simultaneously as you press the throttle. so you sacrifice mileage gained by using a smaller primary for the majority of your driving.