As said above, repair patches on the back will give you strength.  You will need some resin or paint to try and hide the crack on the front. Be it a thin resin coat to hide the crack which is sanded back smooth, or a few coats of lacquer.  Ideally fill the crack in first before going near paints as you may need a few coats to stop it just sinking into the crack.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

Which of those will be the best for repair. I know that one say it's especially for repairs.But is it only curing time difference between two?

If you are in a rush, then the rapid repair is a lot quicker curing and also offers some mechanical advantage in terms of flexural strength under impact and loading. Otherwise the El2 is a good all round laminating epoxy and would still give very good results but with more reasonable pot lives with the 2 hardener options. 

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant