50 amps or 60 amps

go with a 60A if you have modern goodies such as a fuel pump or stereo, or if you plan on putting them on anytime in the future. Single groove or double won't affect you.... unless the extra groove bothers you and you don't want it.
To figure out what A you need add up all the draw on your ES if it's 50A or more, you need a 60. If more than 60 go with 80 etc.