Hmm, so basically, no difference. Why can't there just be an easier way when someone says "this one".
You're going to save 28 candy powering up the level 25 to 30 compared to the level 20. If you have a healthy raid scene where you usually have plenty of people I would do the level 25 for some extra damage in raids. But I only power up when there is a raid boss where I need a particular mon.
Going over 30 is generally gives you diminishing returns unless you have a breakpoint for s specific raid boss in mind.