The following table shows values for class 1 modules of all grades. Everything except operation time scales up with higher classes.
A higher grade module always costs twice as much as one grade lower. If you are low on credits or want to keep your rebuy cost low, this may be important to consider. Especially for the larger controllers.
Grades among all classes have the same operation time, A being the longest and D the second longest. It's an important factor, if you are expending more limpets than you are refining tons (running out of limpets before filling up). This can be true for ships that fire a lot of prospectors and/or run many collectors simultaneously or mining in thin reserves.
This is the same as operation time, but put in relation to the cost of a limpet (101 cr). The rates are so low in any case, that you can completely disregard operation time as an important factor, if you are refining more tons than you can expend limpets (eventually having to eject limpets to make room). For reference, even the cheapest resources sell for at least 300-400 cr/t on average, while the ones you want to be selling sell for two magnitudes higher.
A/B always weigh 4 times as much as D/E of the same class and the same as D/E of a higher class, C landing in the middle. Mass is an important factor to consider if either your FSD or thruster profile have a lot to gain for faster intersystem or normal space travel.
Power draw doesn't rise as dramatically as everything else, never making it an important factor of any kind. But it can make the difference of fitting better defences or not.
B grade has the longest and D grade the shortest range. A longer range gives limpets some more leeway to catch up, if your ship moves between asteroids faster than they do. But they will also spend more time trying to collect far away objects. A shorter range gives you more control over what the limpets collect, which makes it more practical for extraction site and wing mining.
As usual C is nothing to write home about, in this case not even as cost efficient. B is better used by pilots looking to collect a lot of material strewn across a wide area, such as pirates and scavengers. E is the budget option if you happen to be low on credits or power, but for just twice the money and little more power you can get the most focused D grade. A has the very longest operation time, but may be considered too expensive/power hungry/heavy for that benefit.
The following table shows values of grade A modules for each class.
The one takeaway from this is to consider the resources invested for additional active limpets. Not just the credit investment, but also the required mass and sacrificed cargo space (power draw once again doesn't increase as dramatically).
Most of the time you will be better off using multiple smaller controllers, sometimes even in slots they don't quite fit, instead of a single large one. Depending on what slots are available to you, classes 1, 3 and 5 can be a good investment. But I have yet to see a mining application where class 7 is worth the cost, mass and equivalent cargo space. This is mostly down to ships with a class 7 or even 8 slot always having more than enough smaller slots as well.
Always experiment with different configurations to see what yields the best amount of active limpets and cargo space on your ship.