Thanks for the replies!
I decided on the LHR Marriott, the reason being I don't want to be awoken up at 5:30AM when LHR awakes as my flight to Florence doesn't leave till 12:55PM.
As for the rates I work for Marriott so both were showing the Marriott Associate Rate available on the date of travel.
For the bus route looking at the map can I take these buses 726/140/111/285/555/556/557 to the New Road Stop or is there a stop closer? How far is it from the New Road stop to the hotel?