My advice is pretty simple~~ you upgrade whichever mint/ plant you use the most 😛 I guess there is no point upgrading appease mint for eg if you don't use any of the plants under its family~ So have a good think about what is your go to strat in arena and then upgrade that relevant mint.
Your choice of which powermint to upgrade is controlled by EA and they stopped the normal cycling and now roll them out according to their own whims.
Improvemint events are held but not often and there isn't any advance notice so it's a case of wait and see, the latest powermint in store (enchantmint) is only available for 9 days so perhaps they're changing from a monthly schedule to a weekly schedule (plus overlap) which would at least make them come round quicker.
