We flew Virgin to Miami last year and are doing so again this year.
Good sources of information are
V:flyer and
Flightontime
Statistically the service has been much more punctual than the LGW-MCO route, from Flightontime.info in 2009 the average LHR-MIA delay was 17 mins, whereas LGW-MCO was 34 mins
The route has featured newer 747s, but also uses A340-600s (our upcoming flights in Oct/Nov are A340-600s). These have v;port entertainment systems. From a recent look at V-flyer I think 96% of LHR-MIA flights had v;port.
Miami airport itself is hectic, and traffic in the Miami area can be busy, so that is a disadvantage compared to flying to Orlando. That said, once you get onto Florida's Turnpike the drive soon becomes much more relaxing. It is nearly 250 miles, or 4.5 hours drive though, so we tend to stay overnight near the airport or the beach on arrival.
The cost of flights seems always to be cheaper to Miami.
I think for a touring type of holiday, where you would have a car and be travelling around, then Miami is a no-brainer, if you're just going to the Orlando area and maybe wouldn't have a car then probably flying to orlando would make more sense.
Edit had to type v;port as typing v
ort looks cheeky