Best Option
John, without a doubt, you'll be better off with the long shaft. That way you'll be confident that the prop is in the water regardless of conditions. I've had a Merc 8 for four years with good results. Another plus is that when service is required, you're likely to find it easier to get service on your Mercury than with some other brands. That probably varies with your area, but I've heard several war stories from people. By the way, where do you sail? Good luck.