The .325x7 sprocket is the smallest option, .325x8 and 3/8x7 are about the same size, and 3/8x8 and .325x9 are even closer to each other.
With about the same size sprocket, the .325 will put more cutters in the cut, but cutting performance and resistance in the cut will vary with the type of cutters, Stihl RS/RSC beeing the potensially most effective. The .325 will have less room for chip transport, and will be more prone to bogging.
As one of the above posters suggested, there will be a lot of factors entering the picture, so to find the answer you will have to try different options with your saws, your chain of choise, and the wood size and type you are most likely to be cutting.
With bars as long as 20", and anything close to full bar cuts, I believe that 3/8x7 is most likely to perform the best on both saws, maybe .325x7 on the 359.
With 15-16" bars, or for limbing with 18-20" bars (nowhere near full bar cuts !), I would at least want to try 3/8x8 on the 362xp, but not on the 359.