...are Minnie Van drivers Disney cast members?
Yes.
Are they more knowledgeable in getting around Disney property?
Probably yes.
In addition, in certain circumstances, MinnieVans can go where rideshare cannot. Rideshare can take you anywhere public traffic is allowed, but they cannot go places which are restricted only to Disney employees/vehicles. The most important of those is MK, where they can take you to the bus terminal right at MK.
Is there generally a higher level of quality with the higher fare?
Probably.
I drive for both Uber and Lyft in Miami, and I use Uber everywhere I travel, but your reservations are by no means unusual.
I am confident your Uber ride at WDW would be fine -- you should have a newish, nice, CLEAN car and a safe ride from A to B. You will also have a driver who has passed a very rigorous background investigation, and in the case of Uber, continuous background monitoring. Your ride will be double-tracked in real time, and you will have the ability to share the ride progress in real time with trusted family or friends (true also of MinnieVan)
You might have a driver whose English skills are marginal. The Orlando market has a lot of drivers who have fled Puerto Rico (hurricane) and Venezuela (political turmoil, starvation, etc). That doesn't make them bad drivers, but it can make communication challenging. That said, they will be following their GPS and should not have a problem.
Is it worth it?
I just checked AKV to Poly on the Uber rider app and the Uber X fare (up to 4 passengers) was a little less than $11.
MinnieVan pricing is $15 + $2.75 per mile. The trip is 6.6 miles, so the approximate MV fare (for up to 6 passengers) would be $33.15.