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

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?

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

If you're trying to leave the clear finished part surface without doing an ugly repair, just hand laminate 2 plies of the same 200g material over the back with the fibers going +/-45 degrees to the crack.  Extend the patch 50mm past the extents of the crack.  You can do this with a brush and roller and if there's no load on the part (and the forces that initiated the crack are never likely to happen again) drilling a hole will only add a cosmetic issue to fix.  Half a mm of laminate is tough to scarf. 

If you are going to paint the part, do the plies on the back and then once that's cured, lightly grind out the crack (making a scarf joint 30-50 mm beyond the crack like in Hanaldo's picture) and patch with two more plies from the front and then sand/grind it fair once its cured.  You'll want to use normal laminating epoxy (not infusion stuff) for this because it is thicker and will stay in the fabric.  Peel ply helps too if you have it.  You may want to post cure the patch if the part is going to be dark colored and used outdoors.  More on post-curing elsewhere on this forum.

thats a 0.5mm thickness, you may be able to carefully send a bit down on the outside so that the outside patch does not stick out too much, patch it.
Don't forget tho drill the end of the crack, once done on the outside re-enforce the inside.

How do you done finish the part, spray paint or XCR coating resin. The latter will get the best result to hide it best as possible
May be faster and cheaper to do a new part

no problem
depends if you want the repair / crack to show and how thick the carbon fibre is
idealy  I would cover the crack over with the same carbon fibe on the visible side, tonce hardened, sand the inside and patch that up to reinforce it
basically a  double lap as in the picture above

first I would drill a 3 to 4mm hole on the end of the cracks, we did that with acrylic sheets, that helps to stop the crack from extending
I guess it is all about surface, you could patch the surface to repair the appearance, then do a lap repair on the inside to re-inforce it