BA and Virgin both fly to Orlando out of Gatwick - BA from the North Terminal and Virgin from the South.
I haven't flown in economy in either of them for many years but I doubt there is much to choose between them and would go for the best deal, unless prior booking of seats was important to you, in which case Virgin still offers this. With BA this now only happens if you are travelling with young children or have gold card status (or higher).
Stuff happens and you will get people on this forum who had a poor experience with one or the other and would never fly with them again. I've used them both a couple of times each for the last few years in PE and have had good service, 'iffy' food and delays with both, but generally have been quite satisfied - apart from the booking issue with BA, which has now made me switch totally to Virgin.
Both offer online check-in 24 hours before (when you select your seat with BA) and Virgin also offers Downtown Disney check-in.
I don't think anyone uses Heathrow for Orlando - Miami yes. Think your choices of other carriers is limited in January, unless you fly using a hub, eg American Airlines via Chicago or Miami. This would let you choose Heathrow.