Which suject do you enjoy the most? Do you know what you plan to do for a degree? History would definitely be the better option if you are thinking of taking an economics, politics or history degree. However, biology would widen your options. One science isn't useless, but I would say that an A level biology won't prepare you for most scientific careers such as medicine. It does show you have a wide range of skills though. I think both options would work well for you, and -as difficult as it is- the decision will have to be yours and based mainly on which subject you can least cope with giving up. I was struggling between geography and english literature, but eventually chose geography because I thought the workload would be easier, as the other subjects I'm taking will use up a lot of my time. This could be something to consider. Also think about which GCSE you are doing better in.
Sorry I couldn't be of more help
Nooglepop :-)