hmmm..good question. My thoughts is that it will fill up more quickly on the Friday am (and that particular day MK is open later than the rest of the week), althought if you go EE you will for the first few hours have the opportunity to do some rides before it becomes busy...if you are staying on site, that is. On the Saturday, it is my experience that you again need to go early, at opening, for the first few hours and then it will be busy and build throughout the day. MK is open until 11pm that night which will also draw crowds on the Saturday.
It is a bit of a toss up between the two, no clear winner in my mind at least!
Inga