If it is for a parkjet and you already know how to fly, the "F" will do well. You get more punch with the "F" pack. 3S batteries with the "F" power pack make for a pretty zippy power setup.
If you are new and going to put it in a tiny trainer or some other first time plane, go with the "A". The weight and the tameness will save you a lot of headaches. Less weight under less power crashes softer and saves a lot of damage. A 2S with the "A" power pack makes for a great power combo in small, light weight planes.