The reason why your not getting a straight answer is because there isnt one. “it depends” some items get more benefit from decryptors than others do. And different items will benefit more from different decrypters. A lot of items you probably shouldn’t bother with them.
A general rule of thumb is that the more components an item uses, the more benefit it will get from ME decryptors. Smaller items with fewer components that rely on bulk sale will benefit more from max run decrypts.
And certain high value but low volume prints particularly ones with long copy and invention times (looking at you jump freighters) will benefit most from +invention chance.
As always you need to balance out the extra isk you would get using one vs not. Especially since it is a random chance, you may need to use multiple decrypts to get a success. (It’s easier to calculate this in bulk than one job at a time.)
I use specific decrypts for specific jobs. But I also do a decent amount of exploration and so often wind up with a surplus of random ones. I might sell any particularly valuable ones that I get. But the less valuable ones I will just dump into the highest value invention I am doing at the moment.
High input material - marauders, black ops, some t2 cruisers (would have to double check those) +ME decryptors.
Low input material, low invention chance. - exhumed, jump freighters. +invention decryptors.
Low input material, high invention chance, low margins - fighters, drones, certain mods +max runs.
Those are just some general guidelines I use, and there are some differing opinions on which ones are “ideal”