ok, so i am still breeding rotoms. I have a 3IV ditto, but one of the IVs is attack. Would i have higher chances going from 3IV ditto + 4IV rotom to a 6 IV rotom, or going from 2IV ditto + 4 IV rotom to a 5 IV rotom
TL/DR: Depending on the IV spread between both Pokemon, the latter is typically faster.
Not TL/DR: Take for instance the following scenario:
Rotom: 31/31/31/31/x/x
Ditto: x/31/x/x/31/31
Remember that when using the Destiny Knot 5 of the 6 stats will be inherited from the parents. Say speed wasn't inherited this time.
(50% chance to inherit 31 HP, since only one parent has 31) X (100% chance to inherit 31 Att, since both parents have 31) X (50% for Def) X (50% for Sp Att) X (50% for Sp Def) = 6.25% chance to inherit a 31 value for HP through Sp Def.
Since Speed wasn't inherited, you have a 1/32 (or 3.125%) chance of getting a value of 31 (or any other single value for that matter). Thus, the chances of you getting all 31s is: 6.25% X 3.125% = 0.195%.
Now, if Attack wasn't inherited instead of Speed, then the odds would be a bit lower since Attack is the only stat were both parents have 31. It would be 0.50^5 for the first part, rather than 0.50^4. Also consider that this was a scenario where every inherited stat had a 50% chance to be 31. If neither parents have a 31 for a particular stat, then the child will never inherit a 31. An example would be like this:
Rotom: 31/31/31/31/x/x
Ditto: 31/31/31/x/x/x
Still a 4IV Rotom and 3IV Ditto, but this case has a 0% chance of resulting in a 6IV Pokemon. You'll only be able to get all 31s if you use a different parent.