It is much better in that it has faster theoretical up and down speeds. The key word here is "theoretical". That is, when the best conditions are met, the 6 will outperform the 5S. There are many factors here, most of which are going to depend more on the cell tower and your surroundings than anything else.
For simplicity, let's just say 10mbps is 5S max and 20mbps is 6 max (these are made up numbers, by the way). If the cell tower is only able to accommodate 8mbps, the 6 won't see any gains over the 5S, despite it's superior hardware.
Obviously that is very simplified, and you can actually see differences despite a cell tower not performing past theoretical peaks of one hardware over another. I guess what I am trying to say is, we can;t really give a definite no or yes for your area. If you are buying from a major carrier on contract (or even Apple direct), your risks are pretty low; if you don't like the upgrade you can return it.