The equivalent lens would be the 16-35mm f/2.8 or the 17-40mm. Any thing over 24mm you will prefer to use the 24-70mm because it is so much better.

For me I find 24mm wide enough. A few mm less you start seeing lens distortion. I have had a 16-35mm for years, now that I have the 24-70mm I am going to sell it. If the wide lens look is what I wanted I think I will choose one of Canon or Zeiss primes to compliment the 24-70mm.