Go with the A sail for sure
Phil-The biggest difference between the drifter and the asymmetrical spinnaker is that the drifter, being a flatter cut sail will allow you to sail higher - on a closer reach. Assuming that you are cruising with this sail the difference in performance between a drifter and a large genoa barber hauled to the rail will not be so significant that you would want to run right down and buy one. In the light airs that the drifter would be most effective in, most cruisers would likely start to motor. The asymmetrical spinnaker, however, would allow you more flexibility since, based on how full it is cut, it will give you a lot more power on a broader or deeper reach. The fuller the sail is cut the better it will rotate out from behind the main and the deeper you will be able to sail with it. On this point of sail the addition of the a-sail to your inventory would make a significant difference in your performance over your genoa. If you are looking for a racing sail a symmetrical spinnaker would probably be a better choice. Unless you are racing shorthanded where the simplicity of the a-sail is convenient, the symmetrical kite will allow you to sail deeper angles thus giving you more flexibility. Asymmetrical kites on sport boats are so effective because of the boat's great speed potential. The faster they go the more the apparent wind moves forward and the deeper they can sail. A 35-40' racer/cruiser type boat will never see those speeds and will, therefore, not be able to use the a-sail as deep as a symmetrical kite.Good luck.