both. because one of the factors of "the speed at which your city grows" is how far you are away from the total capacity.  as you near capacity, growth slows down.  adding grain increases capacity which can increase growth rate if you're far enough away from that total capacity.  make sense?

which ever one will provide the bigger benefit at the time... if you click on one choice, watch how it modifies your growth (look down in the stats of the town, it will show what kind of benefit that building will bring) ... then click on the other and choose which ever is the greater benefit.

now there are some other things to consider strategically speaking such as should you even care about growth?!?  it all depends on what you're trying to do, how fast do you want to grow?  which city specialization is it?  what does your empire need?  what level is your city?

AFAIK, the only stats which are boosted automatically by city levels are research, production and gildar income (different ratios for different city types).  Grain, materials and essence do not increase automatically.