Wet or Dry set for patio?

For a new home addition I want to use travertine stone on a covered veranda, raised terrace, and a patio surrounding a pool.  There will be about 1200 sf of surface all together.  I'm assuming they will lay the tile on top of a concrete surface with grout, a wet set if I understand the term. 

For the outdoor patio surrounding the pool, am I ok with dry set pavers set on gravel and sand, w a membrane beneath and no grout?  What are the pros and cons of wet/dry set and grout vs no grout? 

 

I'm planning on using 18x18 inch tumbled travertine outside, and maybe a honed tile on the veranda and deck, which will be separated from the pool area by about 6 stairs. 

 

thanks for any advice you can offer,

  Don