Filling gap in concrete
I'd go with the urethane sealant and backer rod. Whole job done for 30 bucks and no mess with cementious stuff, no mixing, easy cleanup (if you don't get it all over you :D)I use Vulkem 116 or Sikaflex for that. Both hold up great for stuff like that. I use Vulkem more because I can get so many colors, but I can't get it at HD, so I use Sikaflex when I can't get Vulkem.
Get the right size backer rod so it goes in and stays, but doesn't get all twisted and jammed in there when you install it. If you want it to last the concave shape front and back is important to let it stretch. Use a plastic spoon to tool the top, and keep a rag with some paint thinner on it handy.