the 6ply tire is more heavy duty (and heavier) and will be more durable. i ordered a set when they first came out and got the 4ply as the 6ply was not yet available and mine have been great and i have yet to get a flat (i also have tire slime in them). when i get my next set i doubt if i will get the 6ply tires as mine have many years on them- mountains, gavel/rock pits, dunes.