Personally, I would not choose to return at Luton airport. The owners of that airport are going out of their way to make that place as un-useable as possible. The road network at the airport is a confusing mess. Then you need a bus to the station - and you can't get a through bus/rail ticket. Don't underestimate the time/hassle of using that vile airport.

Watford would be my choice in this circumstance. The Hertz depot is very near the station. However, you need to be quite early - it closes at noon on Saturday. And it is located down side streets - so if you don't have sat nav it could be a pain to navigate to. Trains to Euston are very frequent and quick - use the LondonMidland services, much faster than Overground.

Alternatively, if you really are not comfortable with navigating in unfamiliar areas, then I would return to London City airport. That sounds counter-intuitive because it's on the wrong side of London for you. However, the airport is very well connected to the motorway network - M1/M25/M11 and you are almost at the airport already. The rental cars at LCY are parked two minutes walk from the terminal building and the walk from the car rental office to the DLR station is one minute. You'll be out of the airport and on a train in no time.

You could also drop at Heathrow, The advantage is it is also well connected to the motorway network. But it is a faff and a half to get from the rental compounds into the airport and to a station - a trundle in a shuttle bus.