I've never had an issue at Philly...
I have nearly always been able to get to the boarding gate on my next flight within one hour of touching of my European flight with US Airways through PHL.
I think it really depends on how busy passport control are... As I know at some stages during the day, a number of European flights all arrive at once.
As you have not transferred through a US airport before off an international flight let me explain the process. You first clear passport control at the first US airport you land at, you then go to the baggage carousel and collect you luggage and clear customs.
Once this is done you check your baggage onto the next flight. This doesn't mean you need to go to a check in, like you would do at Manchester, but you simply hand your luggage to an agent who puts it back on to the belts for its next flight, as it will have been labelled up at Manchester for it final destination of MCO.
As the US has very strict security requirements, you will then need to clear security again like you did at Manchester. All internatonal passengers have to do this, and can sometimes be a little queue here.
I think two hours at PHL is perfectly doable to transfer, but if you do easily get nervous and just want to relax as you go through the airport choose the four hour stopover option, that way you can get a bite to eat and just chill.
Slightly off topic, I know US Airways is usually very competitive with flights to the USA, but have you checked direct flight options to Orlando from Manchester as Virgin fly there and probably be 3-4hrs less travelling, by time you have done the stopover.
Also you have chartered flight options with the likes of Thomas Cook, Monarch and Thomson. I do however think they fly direct to Orlando Sanford, instead or Orlando's main airport. Also the likes of Thomson have greatly improved their legroom and service on long haul flights in recent years and are comparable with the scheduled carriers with seat back TV's etc...
If you are searching on websites like Expedia or Opodo, they don't usually show chartered flight options and have to check these prices through the tour operators own website.