Which Unit to take for the March sitting?

Hi Guys,

I have only done f4, f6 and f8 so far and looking to start my next unit for the march exam sitting.

I was wondering whether out of f5 and f7 which would be the best to start over January being my busiest time of the year at work?

I normally self-study so bearing that in mind I would prefer to start the unit which has less to it?

All advice is appreciated.