Figured I'd add my two cents.
Basic rule of thumb for any tile work is:
use unsanded grout for joints under 1/8"
sanded grout for anything larger then 1/8"
It's not that sanded grout is more durable necessarily but the unsanded grout tends to hold too much water and it will shrink and crack if used in joints larger than 1/8". Almost all mosaic artists recommend sanded grout unless your joints are teeny, teeny. The one thing you do have to be careful of with the "sanded" grout however is if you're using glass tiles the sand
can scratch the glass surface.
Epoxy is recommended for anything that will be staying outdoors year round but it's rarely used unless you're willing to pay as much as 4x the cost of sanded or unsanded grout. I think its most common use is tiling for swimming pools so I guess if you've got enough money to have your pool tiled you don't have to worry about the extra cost!
You should be fine with a couple of coats of penetrating concrete sealer and then reapply over the years for a longer life.
