Fewer bat crazy drivers on the Turnpike than I-95, in my opinion.
With regards to tolls, you don't pay them as soon as you get on the Turnpike, at least that's how it is traveling down on I-75 and then getting on just south of Ocala around Wildwood. I'm going to assume it's the same traveling north from Miami, since the Turnpike starts/ends (depending on the direction you're traveling) in Miami (ok, technically Homestead, but it's still Dade County). According to the map, it looks like the first toll plaza driving north is just north of Delray Beach. There are no tolls on I-95.
I'd take the Turnpike. Get on in Miami and it's a straight shot to the exit for 429. From there, it's only one toll and 20 minutes to Western Way (Exit 8 on 429/192) and Disney World!
I haven't been to Miami in years, but when you pick up your rental car at the airport, ask for printed directions to the Turnpike from the airport. Like I said, you stay on the Turnpike until you see the exit for 429.