I honestly think they should try both (if they think anything at all can work, which I don't necessarily do)! Neither route would really cannibalise the other.
If they want to compete with the train in such a way as people will be willing to pay a fair amount for the pleasure then there needs to be a number of flights per day that are at nice times and that are convenient for as many onward destinations (by surface and air) as possible. It's all very well for people to point out that the train that arrives before 10am is pricey, but there are a lot of trains and it won't necessarily be possible to beat them on convenience with 2 flights a day so an airport that doesn't best serve the largest area of SE England. And if they want to complete for all the passengers that go on all the other much cheaper trains... well, they're not paying so much at all and that might be more of a SEN type job!