Do you want to see nighttime entertainment in the MK? With only a 1 day ticket to MK there are 2 different options:
1. Go to MK on a party day
PRO - crowds will be substantially lower and you will get a lot done in less time. Around 4PM party crowds will enter and lines will quickly grow.
CON - you will not be able to see MK nighttime entertainment
2. Go to MK on a non-party day
PRO - you will be able to see fireworks and nighttime parade (if it is running)
CON - crowds will be substantially higher your entire day and evening
Having experienced both, unless you 'must' see the fireworks and nighttime parade, I'd choose a party day in the MK over a non-party day.