If both have the same main stat, the 6* one would be better (e.g. 5* crit damage gloves- 65% and 6* crit damage gloves -80%) 

But another factor with choosing would be which piece had the better substat procs, and which hero you would be using the piece for.

An example of this would be if I had to choose between to HP% gloves for Arbiter.

     One was a 6* epic gloves with 3 procs on speed (say around 18 speed), and the other was a 5* legendary gloves with 4 procs on speed (around 24)- I'd choose the 5* legendary gloves and trade off 10% HP for 6 more speed.  


The bottom line is, more primary stat usually means better gear but it would still depend on who you would be using the gear for and if you'd need the substats more than the main stat.

Thanks for the reply!

That was very helpful