Help with Universal Annual passes

[Jack&LucysMumy]

Please can someone please answer my questions about Universal's annual passes?

So after reading a thread on here the other day which I can now not for the life of me find again, I began researching purchasing Universal two park annual passes instead of the 14 day passes that we normally buy. Now I'm not actually sure we will visit again next year so I'm basing these tickets on just one trip.
We will be in Orlando from the 21st August- 4th September so it looks like there are no block out dates but can anyone confirm? So my questions are-:
Do any of you guys buy these passes for just one trip?
Can it work out cheaper then 14 day passes especially with the discounts you can get at vendors and hotels etc?
Does anyone buy one preferred pass for the free parking and normal passes for the rest of the family?
Is there anything else I'm missing with regards to it not being a good idea to buy?

Thanks for all of your help