I'd do it in something that either genuinely interests you, or has great employability. I'd also consider that masters fees will be rising this year in reaction to undergraduate fees going up. (the course I teach on is raising their fees a bit this year and then more next year, so we'll charge more than an undergrad year - because then its seen as worth more. hmmmm).
If your looking for an option that will give you employability, you may be better off going into a graduate scheme e.g. PWC, Deloitte, Shell, HSBC etc.