US Airways have actually turned their reputation around in my eyes. About five years ago I flew from Glasgow to New York via Philadelphia and they lost all my luggage, with virtually no compensation (apparently their liability excluded pretty much everything I was carrying, except basic clothes).
However, since then I've been persuaded to take more flights with them based on cost - despite being the last of the big "legacy" airlines to form (through the merger of a number of east coast regional airlines and then the Arizona-based America West) and despite having had some seriously difficult labour relations, they now operate with some of the lowest staffing overheads of the trans-Atlantic airlines, and that means low fares.
All US Airways flights (including their seasonal services from secondary cities like Glasgow) go through their two east coast hubs in Philadelphia and Charlotte. For Florida you could go through either. I like PHL a lot and used it several times. My wife used Charlotte and had a terrible experience at customs (even though she is a US citizen) and swore never again. It could all depend on which agent you get on the day, but I would be minded to prefer PHL over CLT. Once you're in the US and flying domestically, their Airbus fleet is one of the world's largest and youngest.
Regarding the other airlines, for Florida Virgin have lots of direct "sunshine" flights to Florida, but they can usually charge a premium for the convenience of flying direct. Aer Lingus have really upp'ed their game, and the advantage of US Preclearance in Dublin is substantial. It makes arrival after a long flight so much more pleasant.
Delta are ok - just ok, in my experience; you will likely find some good itineraries via the hub they share with KLM in Amsterdam from one of the fifteen or so regional UK airports they serve, but they're not likely to be cheapest.
United (which now includes Continental, with whom they merged a few years ago) have lots of daily direct flights from regional UK cities to Newark. This is another hub worth considering; marginally preferable to transiting in JFK. Like US Airways, maybe United flights from regional UK airports use narrow body Boeing 757 aircraft, which means just three seats either side of a single aisle. Some people prefer a widebody plane (with two aisles and a middle block of seats) but I always hate being stuck in that middle block. The smaller 757 always load and unload quicker and I don't find the single aisle gets congested.
American Airlines - no idea, but the company is having serious labour disputes right now and although they're big in Florida they fly some of the oldest planes domestically and regionally.
Note that one alternative you might not have considered is Air Canada via Toronto or Montréal. They're often cheaper if you carry on to the USA rather than landing in Canada, and you get to pre-clear US Customs in Canada.
To get the best fares, play around with Kayak and Skyscanner, or consider launching a contest on Flightfox.