3 attacks hiting on 4+ wounding on 3+ with a modified AS of 6+ (human, elf, goblin and skaven infantry in other words) will result in 0,83 kills on average for the saurus. this means that they kill almost one enemy more every turn, and thus can grind down their foe quicker. this should be balanced against the fact that if they are deployed 6x4 (compared to 5x5) they will likely not have 3+ rank bonus for very long, and the fact that one extra rank can be important in the long run to deny steadfast.
it is a very tricky choice, and it is very hard to say what is correct. i use troops 6 wide because i want to be able to win the first round of combat decisively, but 5 wide is a very good option too. If you use temple guard or spear sauri, 6 wide should be much more efficient than 5 wide. same goes if you are fighting enemies with T4 or greater.