You could have glorious weather or horrible weather in either of those times. I'm pretty sure that with a bit of googling you could find average weather stats (average hours of sunshine, average mm of rain, etc) broken down by month if you really want to play the odds and maximise your chances.
Bear in mind also that you'll have less daylight in October, particularly after the clocks go back. I'd probably choose April for that reason alone, other things being equal.
But do check the dates of Easter for 2013 if you're thinking of going in April. Prices of accommodation rise dramatically around Easter, which may be something you'll want to bear in mind. They are also very high in the autumn half term, which I believe is sometime in October.