This will always create heat among pilots. The advantages and otherwise are:
1. Heels on the floor expecially during takeoff means far better rudder control and is easier to keep the a/c straight expecially in a strong crosswind. With modern a/c all having an RTO function on the autobrake, the brakes get applied sooner which gives a bit of extra stopping power over manual braking at least at the very beginning of the RTO.
2. Heels off the floor and toes ready on the brakes means faster manual braking if required but in a modern a/c how likeely is it that you'll have both an engine failure and an autobrake failure at the same instant?
As someone already said, it's "horses for courses" unless of course your company SOPs mandate a particular method. It's also important to know your a/c's autobrake design. Does the autobrake high/max setting give actual maximum braking effort that the brakes can attain or will manual braking while literally "standing on them" do better? Even if manual gives more braking effort, at least the RTO function will get the retardation started sooner than any human can and you can then take over manually but be sure to actually use the maximum. Boeing did a survey some years back that showed that the only pilots who actually used FULL MAX brakes in their sim were their own certification test pilots. All others failed to use the a/c to its max potential.
For me its heels on the floor for the above reasons but I know that others have probably equally valid opposing views. Having watched pilots over many years in the sim make one cockup after another of RTOs, I've come to the same conclusion that Boeing did. Thank God for autobrakes and I wish pilots would realise that more people have been killed by mis-handled RTOs than by continued takeoffs with an engine out.