I haven't flown with First Choice but have flown with Virgin every time we have been to Florida, which is a lot
Recently flew Premium Economy both ways with VA and was very impressed. Cabin crew, food, service , all excellent, extra baggage allowance, cases off the plane first, DTD check in meaning no hassle of taking luggage to airport , plus the extra leg room.
The biggest plus point is being sat upstairs in the 'bubble', there is so much room up there, aisles are wider and because the cabin crew just serve the upstairs section the whole feeling and atmosphere is more pleasant.
Downsides? well there is a huge difference between the upstairs seating and downstairs PE seating, especially when you consider it's the same price. Most people who have sat upstairs in PE would agree.
Also flying from Manchester the entertainment system on the MAN fleet is a bit outdated now, and we don't have the luxury of the new leather seats...however overall in my opinion well worth the money.
*edited to add I wish they would include a decent pen in the amenity pack, that is one that doesn't fall to bits when you use it*
I understand the First Choice planes whilst smaller are newer *correct me anyone if I am wrong* and I think their PE has leather seats too.
Personally with my son having special needs I wouldn't be that keen on flying into Sanford, which for us is a major downside, not necessarily for others.
If you have no real problem with Sanford, I would probably decide based on cost, especially as the overall price of a Florida holiday at the moment is very high. Virgin flying direct to MCO is an advantage in our circumstances (they just need to sort out the pen problem!)