I'm afraid this is not how maths works. Hit is not a better stat because it affects more dice rolls (unless you're comparing abilities that trigger on a certain dice roll such as mortal wounds on a six). For a normal attack, hit, wound and save rolls are all equally vital in determining damage.
For example, the chances of damage going through when hitting on 4s (50%), then wounding on 3s (66.66%) followed by a 3+ save (33.33% for an unsuccessful save) is the same as the chance of damage going through when hitting on 5s (33.33%), then wounding on 4s (50%) and then saving on 5s (66.66% chance of unsuccessful save).
In both attacks, there is an 11% chance of the attack being successful - the order of operations does not matter.
Rend does not increase in value if damage goes up. Both rend and hit and wound all remain exactly the same in determining the chance/percentage of the attack going through, no matter what the damage that is being applied at the end is. In the above examples, there is an 11% chance of the attacks going through whether they are damage 1 or 2.
Whether it is better to be buffing hit or buffing rend is dependant upon all that stats involved, so we'd need to see the full hit, wound and save to know whether it was better to buff hit or rend.
For example, if you buff a hit stat from 4+ (50%) to 3 plus (66.66%), then you are increasing the chance of damage being successful by about one third (33%).
If however you were reducing a save from 2+ (16.66% of unsuccessful save) to 3+ (33.33% unsuccessful save) then you've increased the chance of damage going through by double (100%).
If you buff something by one point, then because AoS uses a D6 system, you're always adding one point out of six. In general it is better to chose to buff a weaker stat (like 6+) rather than a strong stay like a 3+(especially given that a 1 is always a fail so no need to buff 2+ stats).
Of course the different factor with rend is that you don't know what save your opponent's army will be and sometimes rend will be useless if the opponent has no save. However, that is balanced out by the fact that within the meta of the game you do need a way of dealing with high armour save units, particularly when they can re-roll their saves and rend is v valuable in doing this.